G3b packet 3a: the seven inbound surfaces a measure undo can strand

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Undoing a CreateMeasure removes it from the owning StaffInstance, reached
through measure_values' carried parent rather than a search. The strand guard
covers seven surface classes, not the two earlier drafts assumed: spanner,
repeat, another measure's start, meter change, system break, page break, and
tempo segment -- the last at both its start and its optional end anchor.

It cannot use the structures index, which is event-only by construction and
would make the guard born green; rewriting it that way breaks fifteen tests.
RepeatStructure is not a CrossCuttingValue variant, so the modify chain
structurally cannot hold one and a repeat_values ledger carries it instead --
seven sites, no delete site. Deleted repeats stay in the map, matching the
ratified retention discipline and making the tombstoned-referencer row
reachable at all.

Owner rules are per surface. Six classes always have one; tempo segments split
by key, since a score-level segment has no owning object to test for liveness.

The five WriteChain surfaces are restoration-aware in both directions, and the
shared path now normalizes a value that does not reference the target to
absent. That is behaviour-preserving -- the guard only ever asks whether a
value names this target -- and it is what lets the two directions fail
independently: spanner chains have no explicit-removal form at all, and a
removed break is still Some((anchor, false)).

Also closes a hole that predates G3b: a minted TimeSignature still named by a
live Measure.time_signature now blocks its own undo.

Executed against spec/CONTRACT_GENESIS_G3B_MEASURE.md pin 10, mutations M48-M63.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01QjsEnYhm1gPpf6ii2iFxFV

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Levi Neuwirth 2026-07-30 18:43:53 -04:00

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