fix(studio): harden composition timeline reliability

[miguel-heygen]

Summary

Composition editing in Studio is now much closer to the CapCut-style behavior Jake and Imad expected: whole comps can be moved, inserted, split, layered, and undone without the editor drifting into per-pixel history or editing the wrong nested layer.

This follow-up PR tightens the stack around the real failure modes from the Day 12 project:

• dragging a comp from the Compositions tab can insert it at the playhead/timeline, with server-owned collision handling and cycle checks
• dragging a comp onto an occupied lane can no longer resolve back to its origin as a silent no-op; it crosses the aimed lane in the intended direction
• successful lane moves reassert their optimistic state and refresh the preview only after the complete lane + z transaction, so stale runtime manifests cannot make the move appear only after a browser refresh
• moving expanded comp children no longer double-applies movement when the host is selected too
• cuts are atomic across touched files, rollback cleanly if a later write fails, and give split composition hosts unique data-composition-id values
• color, font-size, and numeric inspector gestures commit as one history entry instead of hundreds of one-pixel undo steps
• text selection resolves to the paint/text element rather than the transparent mask frame around it
• nested composition playback and playback-rate math stay canonical when trimming, splitting, or moving hosts

QA

• pre-commit hook passed: filesize, lint, format, typecheck, tracked-artifact checks, and fallow gate
• complete Studio suite: 2,703 tests passed (18 todo)
• Studio Server mutation/insertion/routes suite: 87 tests passed
• @hyperframes/studio and @hyperframes/studio-server typechecks passed
• @hyperframes/studio production build passed
• changed-file oxlint, oxfmt, and git diff --check passed
• manual Codex in-browser QA with a clean copy of Day 12 HF Studio Intro.zip:
  • reproduced the stale-lane bug: disk had the correct new data-track-index values while the live timeline kept the old lanes until refresh
  • moved Headline through an occupied lane and verified the live lanes matched disk without a browser refresh
  • performed two more consecutive composition lane moves in the same Studio session; each converged after the transaction and matched the saved tracks
  • verified lane-move Undo and Redo against the saved data-track-index values
  • moved Headline horizontally and restored its authored data-start with one physical Cmd+Z
  • marquee-selected Headline + Lockup, moved both by 0.5s while Background stayed fixed, then restored both with one Cmd+Z
  • Razor-split a composition, Undo/Redo restored both states, and verified the slices use headline / headline-split without StaticGuard errors
  • inserted a colliding Headline composition at the playhead; it received a unique ID and a new track
  • final browser console error log was empty

The in-app pointer automation cannot synthesize a native HTML5 DataTransfer, so native card drag is covered by the Compositions card drag/drop contract tests; the same insertion transaction was exercised in-browser through the card's add-at-playhead action.

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