epiphany/crates/epiphany-core/DECISIONS.md

epiphany-core — decisions and Pass 11 candidates

This file records (a) the implementation decisions the QUICKSTART asked each agent to make once and document, and (b) the ambiguities discovered while building epiphany-core, batched as Pass 11 candidates for the spec rather than improvised in code (QUICKSTART, Process notes: "Ambiguities go into a batch, not into code … Don't open Pass 11 until you have at least three such items batched.").

Implementation decisions (QUICKSTART "Decisions you'll need to make")

1. Replica ID entropy source — getrandom. ReplicaId::generate fills 8 bytes from the platform CSPRNG and re-draws until the value is not the reserved SYSTEM_DERIVED namespace (Chapter 5: "MUST reject this value … and MUST regenerate"). ReplicaId::from_entropy is the deterministic, testable entry point. This is the spec's only sanctioned use of platform randomness (Appendix D §"Randomness").

2. Event-arena storage — slotmap::SlotMap + HashMap<EventId, EventKey>. The slotmap provides generation-checked stale-handle detection (matching the identifier-stability requirement); the hash index provides the required O(1)-amortized lookup by EventId. Canonical iteration (iter_canonical / ids_canonical) sorts by EventId.

3. Chunk store backend — N/A to this crate (Agent D).

4. Async or sync — sync only. No async traits anywhere.

5. MSRV — workspace 1.77 (current stable is used in practice). No exotic features.

Additional local decision: RationalTime's promoted arm uses num-rational::BigRational — the spec's own reference design (Chapter 3 §"Recommended Implementation: Inline-or-Promoted"). Arithmetic takes a fast i128 path for the inline Small ⊕ Small case and promotes only on overflow, re-establishing the "Small iff fits" canonical-form invariant after every operation so demotion is never observable.

Pass 11 candidates (ambiguities for the spec, not resolved in code)

RATIFIED (Pass 11, 2026-06-21). The spec-internal items below have been ratified into normative core_spec.tex text — see spec/PASS11_RATIFICATION_LOG.md. Disposition summary: P11-1/3/6 adopted as golden (TypedObjectId discriminants, promoted-voice + synthetic-pitch derivations); P11-2 fixed (count = 19; three construction-time MUSTs named, and TupletRatio now rejects degenerate ratios at construction); P11-4 adopted (RationalTime/scalar layouts + codec convention baseline the Binary Format companion inherits); P11-7 decided (tempo Linear interpolates speed). P11-5 remains a scope boundary (Track C). The byte-layout golden tests now cite their ratified requirements.

P11-1 — TypedObjectId discriminant values are unspecified

Chapter 5 fixes the shape of TypedObjectId::canonical_bytes ("a 16-bit big-endian discriminant followed by the variant payload's canonical bytes") but does not assign a numeric discriminant to each variant — and its variant list ends with "// … and so on for every named object kind", so the set is explicitly open. Because these bytes enter canonical state (ordering, hashing, equality), the values are normative and must be pinned.

This crate assigns discriminants by declaration order starting at 0 (Event = 0 … AnalysisLayer = 21), and — since TypedObjectId must name every object kind the graph exposes — adds the kinds beyond the spec's explicit list: Tuplet = 22, RepeatStructure = 23, LyricLine = 24, ChordSymbol = 25, View = 26, then Registered = 27. The spec should adopt or override this table and confirm the full kind set.

A sub-point: the canonical bytes of TypedObjectId::Registered(reg, raw) need a defined layout for the registry id. This crate encodes it as discriminant(2) || reg.canonical_bytes(16) || raw_be(16); the spec should confirm the registry-id encoding (and whether ObjectKindRegistryId is a 128-bit value, as assumed here).

Locked (M3 follow-up). The discriminant table and Registered layout are now pinned by a golden-bytes test (typed_object_id_byte_form_is_locked): any reorder, discriminant reassignment, or layout change breaks it deliberately, since these bytes are normative (ordering/hashing/equality). The values remain this crate's proposal until the spec adopts or overrides them.

P11-2 — Graph-invariant count: spec body says 19, QUICKSTART says 18

spec/QUICKSTART.md (Agent B) refers to "the 18 graph invariants enumerated in Chapter 5", but Chapter 5 §"Graph Invariants" actually enumerates 19 items (1–19). This crate implements all 19 (see GraphInvariant). The discrepancy is almost certainly a stale count in the QUICKSTART; the spec body is treated as authoritative. Reconcile the two.

P11-3 — resolved in M2: promoted voices retain the full derivation inputs

Invariant 18 requires a system-promoted voice's VoiceId to equal the deterministic derivation of Chapter 5 §"System-Promoted Voices", whose inputs are (staff instance, original voice, winning op, losing op) — four ids. But The first-pass VoiceOrigin::SystemPromoted recorded only one operation id. M2 resolves the inconsistency by storing { winning_operation, losing_operation, original_voice }; the staff instance remains recoverable from containment.

Invariant 18 recomputes the exact derivation and rejects any SystemPromoted voice whose id does not match it (not merely a wrong namespace) — see check_voice_origin_consistent and the inv18_flags_fabricated_promoted_voice_id_and_accepts_the_derivation test.

The core spec listing now carries both operation ids. The exact hash-domain derivation remains provisional until the semantic-operations companion ratifies derive_promoted_voice_id.

Locked (M3 follow-up). The 64-byte MUSCSVCE preimage — staff_instance || original_voice || winning_op || losing_op, each 16 big-endian bytes — and its hash output are pinned by a golden-bytes test (promoted_voice_id_byte_form_is_locked), so the layout cannot drift unnoticed; the companion's ratification (or a different derivation) will update both the code and that golden.

P11-4 — A prototype canonical encoding precedes the Binary Format companion

Appendix D and Chapter 8 defer the canonical wire encoding of graph value types to the Binary Format companion specification (Agent D), which does not yet exist. To make round-trip serialization testable now (v0 acceptance criterion 4), this crate defines a concrete canonical byte form for its primitives — notably RationalTime (sign + length-prefixed big-endian numerator and denominator magnitudes, always reduced) and the wall-clock integers (little-endian, matching QuantizedCoord).

M3 follow-up — the whole-score codec (item 5). src/codec.rs now composes those primitives into a total, reversible canonical byte form for the entire Score graph (Score::canonical_bytes / Score::decode_canonical), so the materialized graph — not only the Chapter 6 MaterializedState bookkeeping — round-trips byte-identically (Agent F's criterion_4_full_score_byte_roundtrip drives it through a real bundle snapshot). The form is deliberately uniform: little-endian integers, a single discriminant byte per tagged union, u32 counts/length-prefixes, every variable-width leaf length-prefixed, and raw (non-NFC-folded) UTF-8 for free-text fields so decode(encode(x)) == x for every valid score (catalog ids are already NFC at construction). The two private-field accessors the codec needs (EventOrderingDAG::edges_ref, SpellingPrecedence::order_ref) are pub(crate).

These are deterministic and reversible but provisional: when the Binary Format companion lands, reconcile this crate's CanonicalEncode/CanonicalDecode and the whole-score codec with it (a failing cross-crate round-trip test would be the trigger, per the QUICKSTART process notes).

Ratified (2026-07-02): the Binary Format companion now exists (spec/binary_format.tex, v0.1.0). Its Chapter 5 ratifies this crate's whole-Score positional codec, the convention macros, every discriminant table, and the CanonicalValue seam as the schema-major-0 wire form, with the frozen-layout rule (a field-set change is a schema-major change). No reconciliation was needed: the companion was transcribed from this codec and its golden anchors, so the trigger never fired.

Phase 2 — Agent K (Operation Catalog): the CanonicalValue seam. Track B's Operation Catalog shifts epiphany-ops from identifier-only operation payloads to value-typed ones (an InsertEvent carrying the real Event, a RespellPitch carrying the real PitchSpelling). Those payloads must serialize canonically (an envelope stays hashable across an implementation boundary), so epiphany-ops needs to canonically encode/decode core value types. The internal Codec trait and its Reader cursor stay pub(crate) (they are composition machinery, not a stable surface); instead src/codec.rs exposes a thin public CanonicalValue trait (canonical_bytes / decode_canonical) implemented — via a macro that delegates to the existing Codec impls — for exactly the value types operation payloads embed (Event, Rest, PitchSpelling, Tie, Slur, Beam, Spanner, RegionTimeModel, TimeAnchor). This introduces no new byte layout: a value_codec test asserts each value's CanonicalValue bytes equal the bytes the whole-score codec already embeds for it, so all existing goldens / criterion 4 stay byte-for-byte green. This is the K↔J coordination seam: value-type wire encoding is nominally the Binary Format companion's (Agent J) to formalize, but K needs the surface now and J inherits core's ratified conventions (Pass 11 item 1.8, req:format:codec-conventions) rather than reconciling a second codec. Rejected alternative: a parallel value codec inside epiphany-ops (two sources of truth for one byte layout).

P11-5 — Scope boundary: the Chapter 4 tuning catalog is referenced, not defined here

epiphany-core (Agent B) owns the score graph and the pitch/time primitives. It models the identifiers of pitch spaces, tuning systems, and accidental registries (PitchSpaceId, TuningSystemId, …) and the score-level ScoreTuningContext, but not the Chapter 4 catalog itself: the PitchSpace/TuningSystem/AccidentalRegistry definitions, the normative built-in catalog (cmn-12, tet-12, …), the hierarchical resolver, the compatibility mappings, and the deterministic position→frequency resolution function. The QUICKSTART's Agent B deliverable list references those by id; they are a separate subsystem (closer to the acoustic engine, Chapter 1, which is explicitly out of core scope). Consequences:

• Pitch::sounding_equivalent (the third Chapter 2 equivalence) takes a caller-supplied frequency resolver and handles the AbsoluteHz fast path; the other two equivalences are fully computed here. Its tolerance is the named ToleranceClass::AcousticCents (Appendix D forbids ad-hoc epsilons), not a raw f64; a wrong-class or non-finite comparison never matches.
• Tempo conversion integrates the piecewise map in closed form for Constant/Linear/Exponential segments (Chapter 3 §"Conversion"); only TempoShape::Curve is deferred (TempoError::CurveIntegrationUnsupported), per QUICKSTART. The inverse uses a deterministic continued-fraction rational approximation with documented bounds (INVERSION_*). See P11-7.

If a later phase decides the catalog belongs in epiphany-core, it is additive (new modules behind the existing ids); nothing here needs to change. Recorded so the boundary is explicit rather than a silent omission.

P11-6 — System-derived (synthetic) pitch derivation inputs are unspecified

Chapter 5 reserves the SYSTEM_DERIVED replica namespace for deterministically-derived identifiers (system-promoted voices and content-derived synthetic pitches via the MUSCSPCH domain tag) but, as with promoted voices (P11-3), defers the exact derivation function for synthetic pitches. Invariant 11 now requires a SYSTEM_DERIVED embedded PitchId to prove its namespace: its counter must equal the deterministic derivation of its own pitch content, rather than being accepted unconditionally.

Prototype convention (enforced): derive_system_pitch_id content-addresses the pitch from a fixed canonical byte form of its intrinsic identity (scale position + acoustic realization; strings length-prefixed and NFC). The spec should pin the canonical input layout (or define a different derivation).

Locked (M3 follow-up). canonical_pitch_bytes now NFC-normalizes its string fields at the derivation boundary (not merely relying on catalog ids being NFC at construction), making the "NFC" guarantee explicit, and the MUSCSPCH input layout + hash output are pinned by a golden-bytes test (system_pitch_id_byte_form_is_locked). The exact field set ("intrinsic identity") and layout are still this crate's proposal pending spec ratification.

P11-7 — Tempo "Linear" interpolation parameter

Chapter 3 says a Linear segment is "linear interpolation from start_tempo to end_tempo" without pinning what interpolates linearly (bpm, period, or speed). This crate interpolates speed (whole notes per second) linearly, which is beat-unit-agnostic and coincides with linear-bpm when the two tempos share a beat unit. Exponential interpolates speed geometrically. The spec should confirm the parameter (it affects the derived wall-clock schedule).

Enforced-at-construction invariants (Chapter 3 "reject at construction")

Three Chapter-3 well-formedness rules are not in the Chapter 5 §"Graph Invariants" enumeration but are MUSTs the spec says to "reject at construction". This crate enforces them in the constructors (so a malformed value cannot exist), which is both faithful and removes the need for a runtime pass:

• TimeSignature::new rejects beat groups that do not sum to the measure duration.
• EventOrderingDAG::try_new rejects a cyclic aleatoric ordering.
• TupletRatio::new rejects degenerate ratios (either term zero, or actual == notated); its fields are private, so a degenerate TupletRatio is never representable, and codec decode re-validates through the same constructor. (Pass 11 item 3.5 moved this from a runtime invariant-16 sub-check to a construction-time MUST.)

Phase 2 — Agent H (spelling + decomposition pre-passes)

The two sanctioned v0 stubs (pitch::spell returning a trivial middle-C; no decomposition algorithm) are now real, in src/prepass.rs. spell now takes the full &Pitch (was AcousticPitch by value): spelling needs the scale position, which AcousticPitch does not carry, so the old signature could not have done real work — a breaking but necessary change (the only callers were in-crate). The pre-passes are canonical derived annotations (PHASE2_QUICKSTART §H): pure functions of (materialized Score, profile, SpellingAlgorithmId, DecompositionAlgorithmId), recomputed on materialization, never stored. They do not enter the canonical Score bytes — there is deliberately no codec for DerivedAnnotations — so the Chapter-6 reducer and criteria 4/5 are untouched (the conformance suite stays green). derive_annotations(&Score, &PrePassProfile) is the entry point a materializer (F's integration harness) calls after reduction completes.

Phase-2 decisions made (the five the dispatch asked H to make once)

1. Spelling algorithm — Temperley line-of-fifths, registered as SpellingAlgorithmId::default_id() ("default"). A per-voice centre-of-gravity preference rule over the line of fifths (each note picks the tonal-pitch-class spelling closest to a running window of recent spellings, broken by accidental simplicity, then melodic direction, then a total order on the lof value). It is deterministic, key-free (infers tonal context from the melody itself), and spells diatonic music in sharp/flat keys correctly (verified against C/D-major and B♭-major scales and sharp/flat contexts in the prepass::tests suite). Authored CMN scale positions are preserved, not re-spelled (an authored C♯ stays C♯); the algorithm only decides spelling for integer/chromatic (12-EDO) input, which is where spelling is genuinely undetermined. Chosen over Longuet-Higgins line-of-fifths because it is the best-documented and has the cleanest deterministic constraint formulation (PHASE2_QUICKSTART recommendation). Awaits G ratification in Pass 12 (Pass 11 is closed), per the dispatch's "or Pass 12 if the call slips."

2. Decomposition algorithm — metric greedy-aligned splitting, registered as DecompositionAlgorithmId::default_id() ("default"). All grid logic is integer arithmetic over a 1/4096-of-a-whole-note grid, so every note value to a (single-)dotted sixty-fourth is exact and the derivation is deterministic. A duration is split at barlines (with ties), then within a measure each span is emitted as the single notated value it equals unless it would cross a dyadic boundary at least as strong as the one it starts on (the beat-clarity/syncopation rule), in which case it splits at the strongest interior boundary and ties across. Tuplet members convert sounding→notated in the exact rational domain before gridding (a triplet eighth's sounding 1/12 is non-dyadic; its notated 1/8 is), then decompose and carry tuplet membership. Components' sounding durations sum to the event's (invariant 15).

   The remaining three Phase-2 decisions (solver architecture, renderer SVG dialect, catalog/companion versioning) belong to Agents I/K/J, not H.

Eligibility taxonomy

derive_annotations classifies and counts every event and embedded pitch into explicit buckets (TaxonomyReport), so "ineligible" is never silently absent (PHASE2_QUICKSTART §H): pitched events → spelling per pitch + decomposition (if metric, determinate musical duration); rests/unpitched → decomposition, no pitch spelling; trajectory pitches are spelled but the event is not decomposed; graphic/indeterminate/cue → neither; non-cmn-12-determinable pitch spaces → spelling_unavailable; proportional/aleatoric regions → decomposition_deferred_nonmetric.

Precedence rule (H formalizes the rule, not the model)

resolve_spelling layers authored overrides above the inferred default: an engraved-layer, pitch-scoped, Explicit SpellingAttachment whose SpellingSource kind outranks Inferred in the score's SpellingPrecedence wins; otherwise the algorithm's spelling stands. This is the precedence a RespellPitch override rides on. Coordination with K: the v0 RespellPitchOp carries only a ContentHash fingerprint of the new spelling (P11-C1), so an override's spelling value is not reconstructable from a v0 op alone; H's rule operates on the authored SpellingAttachments present on the materialized Score (set by imports/analysis today, by K's real value-typed payloads in Phase 2). The rule itself is value-independent and final.

Pass 12 candidates (batched for F's Pass 12 tracker; ≥3, so the batch opens)

• P12-H1 — Ratify SpellingAlgorithmId::Default = Temperley line-of-fifths v1. The algorithm choice is H's call to propose and G's to ratify; Pass 11 closed before H landed, so this is the first Pass 12 item. Until ratified the id "default" is this crate's proposal (it is not a byte-layout, so nothing golden-locks on it).
• P12-H2 — KeySignatureChange / ClefChange are anchor-only placeholders (Chapter 7 detail deferred). Context-aware spelling therefore infers tonal context from the melody (line-of-fifths centre of gravity) rather than a declared key. A real key-signature/clef content model would let spelling and decomposition honour declared keys/clefs and place natural signs to cancel a key; flagged as a graph-model gap, not improvised here.
• P12-H3 — Chromatic-run convention (ascending = sharps, descending = flats) is only a tiebreak in the centre-of-gravity rule, so an isolated chromatic run with no tonal context may pick the enharmonic the convention would not. A voice-leading refinement is a Pass-12 candidate (the dispatch sanctions deferring hard chromatic cases).
• P12-H4 — Decomposition simplifications: single governing meter per region (multi-meter / mid-region meter changes deferred); region origin assumed to be a barline (anacrusis/pickup deferred); compound-meter (6/8…) beat-group grouping beyond the dyadic default; tuplet nesting and cross-beat tuplet members; double (and higher) augmentation dots — MAX_DOTS = 1 for v1, so a double-dotted value is written as tied single/dotted values (correct, if not the most compact).
• P12-H5 — Automatic spelling under aleatoric regions (the spec's open question). H spells pitches region-independently (pitch identity does not depend on the time model) but performs no region-specific aleatoric spelling; defer if the algorithm does not generalise cleanly.

Audit follow-up (2026-07-01): decomposition precedence + typed inversion tolerance

Authored decomposition attachments outrank the pre-pass (resolve_decomposition)

Audit finding: infer_decompositions never consulted Score.decomposition_attachments, so an authored decomposition was silently shadowed by the derived one — violating Chapter 3 §"Sounding Duration and Notational Decomposition": the pre-pass "produces inferred decompositions for events that lack a higher-precedence attachment", with the "same sources, same precedence machinery, same pre-pass discipline" as spelling.

Fixed by resolve_decomposition, the decomposition analogue of resolve_spelling: for each event the pre-pass inferred a decomposition for, an authored DecompositionAttachment targeting that event whose source outranks Inferred replaces the derived one in DerivedAnnotations.decompositions (the effective attachment keeps its authored source, so provenance is visible and enters the derivation fingerprint). Precedence is the spec's default source order — UserChosen > Imported > Propagated > Inferred — as a fixed rank, because the graph model carries no DecompositionPrecedence configuration and the attachment has no priority/layer axes (unlike SpellingAttachment); among competing authored attachments the lowest rank wins, and a full rank tie keeps the first in the score's canonical (codec-fixed) decomposition_attachments order, so resolution is deterministic across replicas.

Taxonomy decision: authored-override events are counted distinctly in a new TaxonomyReport::decompositions_authored bucket (mirroring spellings_authored); decompositions_inferred now counts only events whose effective decomposition is the pre-pass's own. The effective map size equals decompositions_inferred + decompositions_authored (the H harness's accounting check was updated accordingly). The new bucket is serialized in DerivedAnnotations::canonical_fingerprint with the other counts.

Scope, mirroring spelling: resolution layers overrides above the pre-pass's inferred output only. An authored attachment on an event the pre-pass emits nothing for (ungriddable / non-metric / inapplicable kind) does not surface as a derived annotation — exactly as a spelling attachment on a spelling-unavailable pitch does not (the attachment still lives in canonical Score state either way). Two genuine ambiguities are batched, not improvised:

• P12-H6 — Decomposition precedence configurability. Chapter 3 says "same precedence machinery" as spelling, and Chapter 2 makes the spelling order configurable per score (SpellingPrecedence, plus priority/timestamp tie-breaks); but the graph model has no DecompositionPrecedence field and DecompositionAttachment has no priority. Whether decomposition precedence should be configurable (a new canonical Score field — a codec/ratification change), share SpellingPrecedence, or stay the fixed spec default needs a spec disposition. Until then the fixed default order is implemented.
• P12-H7 — Authored decompositions for events the pre-pass cannot infer for. An authored attachment is precisely how a user would notate an event the algorithm reports ungriddable, yet the derived-annotation surface only resolves overrides where an inferred output exists (the spelling mirror). Whether authored attachments should surface in DerivedAnnotations for inference-ineligible events (and how the taxonomy should count them) is a spec question for both pre-passes.

Typed inversion tolerance (tempo::inversion_tolerance)

INVERSION_TOLERANCE_WHOLE_NOTES was a bare public f64 documented as belonging to tolerance class TempoIntegration but never constructed as a Tolerance (Appendix D §"Tolerance Classes": no ad-hoc epsilons). It is now the private raw magnitude behind the public inversion_tolerance() — a Tolerance { class: TempoIntegration, absolute: 1e-6, relative: None, governance: Validation } (the same construction pattern as the existing speed_degeneracy_tolerance). Behavior is numerically identical: the inverse conversion passes inversion_tolerance().absolute.get() (exactly 1e-6) to the continued-fraction approximation. Note the class's non-normative unit label is "wallclock seconds" while this residual is measured in whole notes; the class identity (TempoIntegration: conversion residual in either direction) is what is normative.

Pass 12 G-pass (2026-07-07): the H rows are ratified

All seven H rows are retired (dispositions in spec/PASS12_RATIFICATION_LOG.md, "G-pass tranche"; worklist spec/PASS12_WORKLIST.md). Summary: H1 "default" = Temperley line-of-fifths v1 is ratified normative (req:pitch:spelling-algorithm); H3 convention-as-tiebreak and H5 region-time-model-independence are pinned as properties of that versioned algorithm; H4 the five decomposition bounds are the declared normative bounds of "default" v1 (req:time:decomposition-algorithm; C5's derived-notation gap is subsumed); H6 decomposition precedence is ratified FIXED (not configurable — a configurable order would be a schema-major Score field with no consumer); H2 narrowed (the content model landed in I-0; key-aware spelling is algorithm-v2 territory, cancelling naturals a notation refinement); H7 decided the other way from the implementation: authored attachments on inference-ineligible targets MUST surface in derived annotations (req:pitch:authored-uninferred) — the code change lands with the G-pass code tranche (authored-only resolution paths + distinct taxonomy buckets in both pre-passes). Also ratified here: system-derived intrinsic content is immutable under reduction (P12-K3; core Ch5 states it, the catalog pins the precondition, epiphany-ops implements).

G-pass follow-up (2026-07-07): unsupported algorithm ids now ERROR

A post-commit review found the ratified requirement and the implementation disagreeing: req:pitch:spelling-algorithm / req:time:decomposition-algorithm say a profile requesting an unregistered id MUST error, but derive_annotations still implemented the pre-ratification behavior (that pre-pass "derives nothing" while the requested id stays in the result profile — the honest-cache-key rationale), and a test locked it. The MUST-error contract is the right one and stands: a silently-empty derivation is indistinguishable from a legitimately empty score, and an implementation that does support the requested algorithm would produce real annotations while this one quietly produced none — two "successful" results that disagree. Fixed: derive_annotations returns Result<DerivedAnnotations, PrePassError> (UnsupportedSpellingAlgorithm / UnsupportedDecompositionAlgorithm, rejected up front); every production caller uses the default profile and .expects; the stale test is rewritten as unknown_algorithm_ids_error. CONFORMANCE.md's long-standing "errors" claim is now true rather than aspirational.

Schema major 2, Phase B: the snapshot side (data-model fills + frozen v1)

The nine type bodies fill to the ratified Ch5 shapes (Binary Format §Schema Major 2): Slur/Tie/Beam/Spanner (kind/curvature/sub-beams/geometry/style — one shared SpanStyle), RepeatStructure (kind/voltas), Staff (default_clef), StaffLineConfiguration (spacing/style/bracket), Instrument (six fields), ScoreMetadata (six fields incl. the strictly-authored timestamps). All new leaf types live in graph.rs with the ratified wire discriminants in codec.rs (tag_only_codec! for the tag-only enums).

The frozen-form architecture generalized: major 2's fills reach types the v0 walk had treated as "unchanged" (metadata, staves, cross_cutting, and — transitively through Region.content — the staff instances), so the frozen wire forms are now a shared sub-codec layer: enc_/dec_*_v1 functions (v0 == v1 for every type major 2 changed) used by BOTH decode_v1_score/encode_v1_score (new) and the v0 pair (updated to route through them). Each versioned decoder stays strictly canonical over its own wire form (re-encode-and-compare, the fuzzer-P1 discipline) and the fuzzer corpus gained genuine-v1 forms + the major-2 seam. The v1_score_migrates_default_filling_the_major_2_fields size anchor pins that v1 omits exactly the appended default bytes (so the frozen encoder cannot silently drift), and current_major_round_trips_non_default_values_for_every_major_2_field exercises every new field (and every payload-carrying SpannerKind variant) as real wire content.

Deliberate scope choices: generator fixtures were NOT given non-default v2 content — that would churn the render goldens and reference-suite metrics for zero coverage the codec tests don't already provide; ops-level coverage arrives when Phase D's valuegen builders emit v2 values. Score::empty keeps its signature (Timestamp(0) is the ratified unset convention; a creation-time builder waits for a producer, e.g. import).

Schema major 2, Phase D — repeat coverage lands; the anchor-site walk gets one home

The Phase-B promise above ("ops-level coverage arrives when Phase D's valuegen builders emit v2 values") is discharged: epiphany-ops::valuegen gained event_anchor/repeat_structure/volta_repeat, the op generators emit the pair, and the decode fuzzer's corpus gains valid_score_rich_with_repeats (DalSegno + voltas) — corpus-local, not in the shared valid_score_rich: the shared fixture feeds the render goldens, and repeat rendering is deliberately E1's churn, not D's (the zero-golden-churn discipline).

RepeatStructure::anchor_sites()/anchor_sites_mut() (graph.rs) are now THE site-set walk (start/end, kind jump targets, volta spans). Review found the set hand-rolled in five places across three crates — and a sixth, indexes.rs, silently stale since Phase B (it indexed only start/end, missing every kind/volta anchor, contradicting its own doc). All flat walks now consume the method (the classified per-site invariant check keeps its exhaustive match for message attribution); the index gap is regression-locked in indexes_build_and_answer_queries.

The interval algebra, and the type that already existed (Push 4a, 2026-07-09)

req:pitch:transposition (core Ch2, §"Transposition and the Interval Type") pins the action of a TranspositionInterval { d, c } on a PitchSpacePosition::Cmn. With n the nominal's normative discriminant and s = nominal.chromatic() + alteration + 12*octave:

nominal'    = CmnNominal((n + d).rem_euclid(7))
octave'     = octave + (n + d).div_euclid(7)
alteration' = (s + c) - (nominal'.chromatic() + 12*octave')

The diatonic component alone picks the nominal and octave; the alteration absorbs the residue. C4 + (7, 12) = C5, not "C with twelve sharps". C4 + (0, 1) = C#4, so the editor's sharpen keeps its exact current behaviour.

The type was already here. TranspositionInterval has lived in graph.rs since schema major 2, carrying Instrument.transposition (a B-flat clarinet is -1 diatonic, -2 chromatic), already codec'd, already exported. It is byte-for-byte the pair transposition needs, so Push 4a reuses it rather than minting an Interval beside it. The spec now lists it once, in Chapter 2; the Chapter 5 Instrument block references that listing instead of repeating it. Two normative listings of one struct is the drift the P13-I1 fix just closed.

Its doc claimed it was "ADVISORY until the Chapter 4 tuning catalog pins interval algebra". That was the P12-K2 false coupling, repeated. Transposition acts on scale_position; tuning acts on acoustic; the two never touch. The algebra is pinned here with no tuning catalog anywhere in sight. What remains genuinely unimplemented is the automatic application of an instrument's interval at the written/sounding boundary — nothing respells a written part into a sounding one — so Instrument.transposition stays advisory, for that reason and not the stated one.

Refusal, not saturation. alteration and octave are i8. A transposition whose result does not fit refuses; so does one against a non-Cmn position (no nominal to move) or an AcousticRealization::AbsoluteHz pitch (which overrides the tuning system, so moving the scale position moves the notehead without moving the sound). Saturation is the worst possible failure here because it is invisible: it produces a pitch nobody asked for, reports success, and destroys the evidence. See epiphany-ops/DECISIONS.md §"Push 4a" for the operation-level consequences and the frozen Transpose.