Precomp Phase F: persistence (schema v29) + edit propagation #106

Phase F of the Precomps epic (#100, ADR-0029). Depends on Phase E (#105).

Scope

1. Serializer refactor first (format-neutral): factor the per-layer save dispatch (ProjectSerializer.cpp:404-740) and load dispatch (:1746-2262) into saveLayerJson(...) / loadLayerJson(...) helpers parameterized on the target Timeline; existing round-trip tests guard it. Land as its own commit.
2. v29 additions (PROJECT_VERSION 28→29, changelog entry in ProjectSerializer.hpp header block):
   • Top-level "precompDefinitions": [ { id, name, canvasWidth, canvasHeight, frameRate, duration, tracks: [ { name, layers: [<shared layer shape>] } ] } ], loaded before the timeline so instances resolve.
   • Instance layer: "kind": "precomp" + definitionId (+definitionName for diagnostics), innerStartFrame, speed, playbackMode (string enum per bus rules), plus shared placement/MediaLayer/Transform/routing-by-name/EffectChain/AnimatedProperties fields.
   • Missing definition on load ⇒ instance disabled + logged, kept in file (never dropped).
   • Shadows and materializedVersion never persisted — full rematerialize on load (definitions load at version 1, instances at 0).
3. Edit propagation polish: complete the PrecompLibrary::touch hooks — CommandDispatcher post-execute (PrecompMember mutations), TimelineWidget edit-commit for definition-bound timelines, definition-level edits (canvas/duration/track ops → canvas change must trigger RT resize-in-place via the snapshot dims). Undo of a master edit must also propagate (undo restores component state → touch → resync).

Tests

• scripts/integration/precomp_persistence_save.json / precomp_persistence_load.json — CMake fixture pair (FIXTURES_SETUP/FIXTURES_REQUIRED, model: oa_layer_persistence_*): save definition + 2 instances (one Loop + speed 2.0, one with an effect chain); load half asserts definition fields, instance params, shadow counts, and a post-load render hash.
• precomp_edit_propagation.json: two instances; trim a master clip via command; assert both instances resync (shadow counts + changed render hash); Undo; assert propagation back.
• Backward-compat: load a v28-era project fixture — must load clean with zero precomp keys.

Acceptance

• Round-trip + propagation + undo-propagation green; v28 files load unchanged; local ctest green.