feat(core): structural insert — insertNodes + insertTimeline verbs (0.6.41)

[kiyeonjeon21]

What

Completes the structural-editing CRUD surface (add / remove / reorder / insert). Insert is the last Spine 2 op; unlike reorder/remove (which patch existing addressable elements) it creates elements, so it needs new compose verbs. Scoped to the generic verbs only (per the roadmap's deprioritize-engine-primitives call) — the montage shot-payload generator is out of scope.

How

• insertNodes — insert a node at a position (before/after a sibling root id, or index) vs addNodes appending on top, so a new layer can land under later nodes (a montage shot below the vignette/scrim grade).
• insertTimeline — splice a step/beat into a named beat ({ into, before/after/index, step }); validates the step's tween/motionPath targets; unknown into/before/after or a missing target is an orphan.
• Montage flatten — photoMontage now groups the shot beats directly under the "montage" beat (dropped the redundant inner seq wrapper — timing-equivalent, golden-safe), so the play order is addressable as into:"montage". Reorder/removeTimeline unaffected.
• The overlay carries the full node + beat JSON (a consumer like reframe-studio authors it; reframe doesn't generate the shot payload).

Verification

• 391 tests pass (+4 insert tests, incl. an end-to-end montage insert: hand-authored shot lands in play order between shot-1 and shot-2, paints under the grade, 0 orphans); typecheck clean; goldens byte-identical (montage isn't golden; flatten is timing-equivalent)
• verify-overlay … montage-insert.json → 2 applied / 0 orphaned; montage-restructure.json still 4 / 0
• Rendered the overlay'd montage: the inserted shot shows full-frame in sequence, under the grade
• reframe labels rows unchanged by the flatten

Notes

• Out of scope: montageInsertShot payload generator; node-insert into a nested group (root covers montage); exact neighbour-crossfade recompute across an insert (cosmetic, same tradeoff as remove).
• Version → 0.6.41 (PATCH).

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