Agent I-0: real clef + key-signature data in the core graph

Prerequisite for the visible-slice engraving milestones (I-1+): give the
score graph the clef/key data the engrave pipeline needs for clef-relative
staff positioning and key-signature rendering, replacing the Chapter-7
TimeAnchor-only placeholders.

epiphany-core:
- ClefShape { G, F, C, Percussion } — each family's reference pitch (G4 / F3 /
  middle C4) pins the staff-position mapping.
- Clef { shape, line, octave_shift } — the SMuFL family, the staff line its
  reference pitch sits on (1 = bottom line), and an octave transposition; with
  treble/bass/alto/tenor constructors and a treble Default. Generalizes to any
  C-clef line and octave-transposing clefs.
- KeySignature — circle-of-fifths position, validated -7..=7: a private field
  behind KeySignature::new (Option) + fifths(), and a custom Codec that rejects
  an out-of-range count on decode (Reconstruct, as PowerOfTwo / Tempo do).
- ClefChange now carries clef: Clef; KeySignatureChange carries key: KeySignature.
- Codec (cstyle_enum_codec / struct_codec / the validating KeySignature impl),
  lib.rs re-exports, a round-trip test over every shape x line x octave and every
  fifths value, and an out-of-range decode-rejection regression test.

Zero blast radius: nothing constructed these with data (clef/key sequences are
always empty) and invariants only read .anchor, so existing Score bytes, hashes,
and goldens are unchanged. Fixtures get populated and the engrave pipeline reads
the data in I-1.

Gates: build/fmt/clippy -D warnings clean; cargo test --workspace green;
conformance_suite scale 1 passes. Stages only epiphany-core; the pre-existing
Agent-I working tree (engrave/layout-ir/render-svg + .gitignore) stays unstaged.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

crates/epiphany-core/src/codec.rs

@@ -48,15 +48,16 @@ use crate::event::{
 use crate::graph::{
     AleatoricAnchoringDiscipline, AleatoricTimeModel, AnalysisLayer, AnalyticalAnnotation,
     AnnotationAnchor, BarlineAlignmentGroup, BarlineAlignmentMember, Beam, BeatGroup, Canvas,
-    ChordSymbol, ClefChange, Comment, CrossCuttingRegistry, DecompositionAttachment,
-    DecompositionSource, EventOrderingDAG, GestureAnchoring, GraphicContent, GraphicGesture,
-    GraphicObject, Instrument, KeySignatureChange, LyricLine, Marker, Measure,
-    MeasureNumberVisibility, MeterChange, MetricGrid, MetricTimeModel, NotatedComponent, NoteValue,
-    PartDefinition, PowerOfTwo, ProportionalTimeModel, Region, RegionContent, RegionTimeModel,
-    RepeatStructure, Score, ScoreMetadata, ScoreTuningContext, Slur, Spanner, Staff,
-    StaffBasedContent, StaffExtent, StaffGroup, StaffGroupKind, StaffInstance,
-    StaffLineConfiguration, StemDirection, TempoMapReference, Tie, TieClass, TimeExtent,
-    TimeSignature, TimeSignatureDisplay, Tuplet, TupletRatio, ViewDefinition, Voice, VoiceOrigin,
+    ChordSymbol, Clef, ClefChange, ClefShape, Comment, CrossCuttingRegistry,
+    DecompositionAttachment, DecompositionSource, EventOrderingDAG, GestureAnchoring,
+    GraphicContent, GraphicGesture, GraphicObject, Instrument, KeySignature, KeySignatureChange,
+    LyricLine, Marker, Measure, MeasureNumberVisibility, MeterChange, MetricGrid, MetricTimeModel,
+    NotatedComponent, NoteValue, PartDefinition, PowerOfTwo, ProportionalTimeModel, Region,
+    RegionContent, RegionTimeModel, RepeatStructure, Score, ScoreMetadata, ScoreTuningContext,
+    Slur, Spanner, Staff, StaffBasedContent, StaffExtent, StaffGroup, StaffGroupKind,
+    StaffInstance, StaffLineConfiguration, StemDirection, TempoMapReference, Tie, TieClass,
+    TimeExtent, TimeSignature, TimeSignatureDisplay, Tuplet, TupletRatio, ViewDefinition, Voice,
+    VoiceOrigin,
 };
 use crate::ids::{
     AnalysisLayerId, AnalyticalAnnotationId, BarlineAlignmentGroupId, BeamId, ChordSymbolId,
@@ -1453,8 +1454,27 @@ struct_codec!(MeterChange {
 struct_codec!(MetricGrid { meter_sequence });
 struct_codec!(ProportionalTimeModel { duration });
 struct_codec!(MetricTimeModel { meters, tempo });
-struct_codec!(ClefChange { anchor });
-struct_codec!(KeySignatureChange { anchor });
+cstyle_enum_codec!(ClefShape {
+    0 => G,
+    1 => F,
+    2 => C,
+    3 => Percussion,
+});
+struct_codec!(Clef {
+    shape,
+    line,
+    octave_shift
+});
+impl Codec for KeySignature {
+    fn enc(&self, out: &mut Vec<u8>) {
+        self.fifths().enc(out);
+    }
+    fn dec(r: &mut Reader<'_>) -> Result<Self> {
+        KeySignature::new(i8::dec(r)?).ok_or(ScoreDecodeError::Reconstruct("KeySignature"))
+    }
+}
+struct_codec!(ClefChange { anchor, clef });
+struct_codec!(KeySignatureChange { anchor, key });
 struct_codec!(Measure {
     id,
     start,
@@ -2193,6 +2213,72 @@ mod tests {
         );
     }
 
+    #[test]
+    fn clef_and_key_signature_codecs_round_trip() {
+        use crate::graph::{Clef, ClefChange, ClefShape, KeySignature, KeySignatureChange};
+        use crate::time::{TimeAnchor, WallClockTime};
+
+        fn rt<T: Codec + PartialEq + core::fmt::Debug>(v: &T) {
+            let mut out = Vec::new();
+            v.enc(&mut out);
+            let mut r = Reader::new(&out);
+            let decoded = T::dec(&mut r).expect("decodes");
+            assert_eq!(&decoded, v, "round-trip changed the value");
+        }
+
+        for shape in [
+            ClefShape::G,
+            ClefShape::F,
+            ClefShape::C,
+            ClefShape::Percussion,
+        ] {
+            rt(&shape);
+            for line in [-3i8, 1, 2, 3, 4, 5] {
+                for octave_shift in [-1i8, 0, 1] {
+                    rt(&Clef {
+                        shape,
+                        line,
+                        octave_shift,
+                    });
+                }
+            }
+        }
+        for shape_clef in [Clef::treble(), Clef::bass(), Clef::alto(), Clef::tenor()] {
+            rt(&shape_clef);
+        }
+        for fifths in -7i8..=7 {
+            rt(&KeySignature::new(fifths).expect("fifths is in range"));
+        }
+        let anchor = TimeAnchor::WallClock {
+            time: WallClockTime(0),
+        };
+        rt(&ClefChange {
+            anchor: anchor.clone(),
+            clef: Clef::bass(),
+        });
+        rt(&KeySignatureChange {
+            anchor,
+            key: KeySignature::new(-3).expect("fifths is in range"),
+        });
+    }
+
+    #[test]
+    fn key_signature_rejects_out_of_range_fifths_on_decode() {
+        let decode = |fifths: i8| {
+            let mut bytes = Vec::new();
+            fifths.enc(&mut bytes);
+            KeySignature::dec(&mut Reader::new(&bytes))
+        };
+        assert!(decode(-7).is_ok());
+        assert!(decode(7).is_ok());
+        for fifths in [-8i8, 8] {
+            assert!(
+                matches!(decode(fifths), Err(ScoreDecodeError::Reconstruct(_))),
+                "out-of-range fifth count {fifths} must be rejected"
+            );
+        }
+    }
+
     #[test]
     fn generator_scores_round_trip() {
         for seed in 0..200u64 {

crates/epiphany-core/src/graph.rs

@@ -51,17 +51,116 @@ impl Default for StaffLineConfiguration {
     }
 }
 
-/// A clef placed at a point in a staff instance. Placeholder (Chapter 7).
+/// The SMuFL clef family a [`Clef`] draws from. The reference pitch each family
+/// fixes (G4 / F3 / middle C4) is what pins the staff-position mapping.
+#[derive(Copy, Clone, PartialEq, Eq, Hash, Debug)]
+pub enum ClefShape {
+    /// G clef (treble family) — reference pitch G4.
+    G,
+    /// F clef (bass family) — reference pitch F3.
+    F,
+    /// C clef (alto / tenor family) — reference pitch middle C (C4).
+    C,
+    /// Unpitched percussion clef — no diatonic reference.
+    Percussion,
+}
+
+/// A clef: the SMuFL [`ClefShape`], the staff line its reference pitch sits on
+/// (`1` = the bottom line of the staff, counting up), and an octave
+/// transposition (`-1` for treble-8vb, `+1` for treble-8va, …). The shape's
+/// reference pitch on `line` fixes where every pitch under this clef lands on
+/// the staff.
+#[derive(Copy, Clone, PartialEq, Eq, Hash, Debug)]
+pub struct Clef {
+    pub shape: ClefShape,
+    pub line: i8,
+    pub octave_shift: i8,
+}
+
+impl Clef {
+    /// Treble clef — G clef on line 2.
+    pub const fn treble() -> Self {
+        Clef {
+            shape: ClefShape::G,
+            line: 2,
+            octave_shift: 0,
+        }
+    }
+    /// Bass clef — F clef on line 4.
+    pub const fn bass() -> Self {
+        Clef {
+            shape: ClefShape::F,
+            line: 4,
+            octave_shift: 0,
+        }
+    }
+    /// Alto clef — C clef on line 3.
+    pub const fn alto() -> Self {
+        Clef {
+            shape: ClefShape::C,
+            line: 3,
+            octave_shift: 0,
+        }
+    }
+    /// Tenor clef — C clef on line 4.
+    pub const fn tenor() -> Self {
+        Clef {
+            shape: ClefShape::C,
+            line: 4,
+            octave_shift: 0,
+        }
+    }
+}
+
+impl Default for Clef {
+    fn default() -> Self {
+        Clef::treble()
+    }
+}
+
+/// A key signature as a position on the circle of fifths: `fifths` sharps when
+/// positive (`1` = G major, one sharp), flats when negative (`-1` = F major,
+/// one flat), and `0` for C major / A minor. The accidental set the renderer
+/// draws is derived from this count.
+#[derive(Copy, Clone, PartialEq, Eq, Hash, Debug, Default)]
+pub struct KeySignature {
+    fifths: i8,
+}
+
+impl KeySignature {
+    /// Lowest key-signature fifth count in conventional CMN notation: seven flats.
+    pub const MIN_FIFTHS: i8 = -7;
+    /// Highest key-signature fifth count in conventional CMN notation: seven sharps.
+    pub const MAX_FIFTHS: i8 = 7;
+
+    /// Builds a key signature, rejecting values outside the conventional
+    /// `-7..=7` circle-of-fifths range.
+    pub const fn new(fifths: i8) -> Option<Self> {
+        if fifths >= Self::MIN_FIFTHS && fifths <= Self::MAX_FIFTHS {
+            Some(KeySignature { fifths })
+        } else {
+            None
+        }
+    }
+
+    /// The circle-of-fifths position (`-7..=7`).
+    pub const fn fifths(self) -> i8 {
+        self.fifths
+    }
+}
+
+/// A clef placed at a point in a staff instance (Chapter 7).
 #[derive(Clone, PartialEq, Eq, Debug)]
 pub struct ClefChange {
     pub anchor: TimeAnchor,
+    pub clef: Clef,
 }
 
-/// A key-signature change at a point in a staff instance. Placeholder
-/// (Chapter 7).
+/// A key-signature change at a point in a staff instance (Chapter 7).
 #[derive(Clone, PartialEq, Eq, Debug)]
 pub struct KeySignatureChange {
     pub anchor: TimeAnchor,
+    pub key: KeySignature,
 }
 
 /// Whether and how a measure number is shown. Placeholder (Chapter 7).

crates/epiphany-core/src/lib.rs

@@ -99,9 +99,9 @@ pub use event::{
 pub use graph::{
     derive_promoted_voice_id, AleatoricAnchoringDiscipline, AleatoricTimeModel, AnalysisLayer,
     AnalyticalAnnotation, AnnotationAnchor, BarlineAlignmentGroup, BarlineAlignmentMember, Beam,
-    BeatGroup, Canvas, ChordSymbol, ClefChange, Comment, CoordinateDiscipline,
+    BeatGroup, Canvas, ChordSymbol, Clef, ClefChange, ClefShape, Comment, CoordinateDiscipline,
     CrossCuttingRegistry, DecompositionAttachment, DecompositionSource, EventOrderingDAG,
-    GestureAnchoring, GraphicContent, GraphicGesture, GraphicObject, Instrument,
+    GestureAnchoring, GraphicContent, GraphicGesture, GraphicObject, Instrument, KeySignature,
     KeySignatureChange, LyricLine, Marker, Measure, MeasureNumberVisibility, MeterChange,
     MetricGrid, MetricTimeModel, NotatedComponent, NoteValue, PartDefinition, PowerOfTwo,
     ProportionalTimeModel, Region, RegionContent, RegionTimeModel, RepeatStructure, Score,