feat(core): structural montage editing — reorder + remove (0.6.40)

[kiyeonjeon21]

What

Spine 2c — structural editing of montages, regen-surviving. Clip-ripple (0.6.39) made retiming survive AI regeneration via label-anchored video.start; this makes structure survive too. A human reorders or drops a montage shot in an overlay JSON and the edit re-applies after the base is regenerated.

How

1. Montage rewrite — self-contained named shot beats (packages/core/src/montage.ts)
Each shot is now a named beat shot-${i} that owns only its own layer's motion (fade-in ∥ Ken Burns ∥ fade-out). Every layer starts at opacity: 0; adjacent shots overlap by the crossfade via a negative gap in the seq. No shot references its neighbour, so it can move/drop as a unit. Stable shot-${i}/cross-${i} labels + node ids preserved → clip-ripple and anchored titles keep resolving.

2. New generic compose verb removeTimeline (packages/core/src/compose.ts)
Splices a beat/step out of its parent by label (parallel to removeNodes); the seq re-accumulates so later steps ripple up. Unknown label → orphan, never a silent drop.

3. The three edits, all overlay-time + regen-surviving

• Reorder → existing beat order patch (no new code)
• Remove → removeTimeline: ["shot-2"] (the dropped layer just stays invisible)
• Swap image → plain nodes.<id>.src patch

Verification

• 386 tests pass, typecheck clean, npm build clean, goldens byte-identical
• verify-overlay examples/scenes/photo-montage.ts examples/overlays/montage-restructure.json → 4 applied, 0 orphaned
• reframe labels confirms each shot-i starts 0.70s (the transition) before the prior ends — true crossfade overlap
• Rendered a crossfade frame: two photos cleanly blended, title + grade intact

Notes

• Cosmetic limitation (documented in regen-contract.md): reordering a shot to the first slot drops its opening fade-up (the crossfade offset was baked for the original order).
• A node still anchoring to a removed label (e.g. a video start: "shot-2") must also be neutralised (patch start to a number) or post-compose validation rejects the dangling anchor — documented.
• Out of scope (separate increment): insert a shot (splice-into-parent + node-add + framing seed).
• Version → 0.6.40 (PATCH); no plugin bump.

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