v0.6.41 — Structural insert — `insertNodes` + `insertTimeline` overlay verbs (Spine 2c)

Added

Structural insert — insertNodes + insertTimeline overlay verbs (Spine 2c)

• Two positioned-insert compose verbs complete the structural-editing CRUD surface
  (add / remove / reorder / insert):
  • insertNodes — insert a node at a position (before/after a sibling root id,
    or an index) instead of appending on top like addNodes, so a new layer can land
    UNDER later nodes (e.g. a montage shot below the vignette/scrim grade).
  • insertTimeline — splice a step/beat into a named beat ({ into, before/after/index, step }). The step's tween/motionPath targets must exist; unknown into/before/after
    or a missing target is reported as an orphan, never a silent drop.
• photoMontage/videoMontage now group the shot beats directly under the "montage"
  beat (the redundant inner seq wrapper is gone — timing-equivalent), so the play order is
  addressable: insertTimeline { into: "montage", after: "shot-1", step } splices a shot into
  the sequence. Reorder (order) and removeTimeline are unaffected.
• Unlike reorder/remove (which patch existing addressable elements), insert creates
  elements, so the overlay carries the full node + beat JSON — a consumer (reframe-studio) or
  the author supplies it; reframe does not generate the montage shot payload. See
  examples/overlays/montage-insert.json and docs/guides/regen-contract.md.