feat(core): GSAP-aware split engine for timeline clip splitting (#1330)

* refactor(studio): extract shared timeline components and deduplicate code

Extract shared utilities to reduce duplication across timeline components:

- PlayheadIndicator: shared playhead rendering (was duplicated in
  TimelineCanvas and TimelineEditorNotice)
- useContextMenuDismiss: outside-click/Escape dismiss pattern (was
  duplicated in ClipContextMenu and KeyframeDiamondContextMenu)
- TimelineCallbacks: shared callback interfaces for drop and edit
  operations (was duplicated in NLELayout and Timeline props)
- useTimelineZoom: consolidated zoom store selectors
- timelineElementSplit: shared canSplitElement, buildPatchTarget, and
  readFileContent utilities
- gsapParser.test-helpers: shared test utilities for parser specs

* feat(core): GSAP-aware split engine for timeline clip splitting

Add splitAnimationsInScript to the GSAP parser — correctly re-times
animations when a timeline clip is split at an arbitrary position:

- Animations before split: kept on original, properties inherited via
  tl.set inserted before other tweens for correct GSAP state recording
- Animations after split: retargeted via AST selector update
- Spanning animations: trimmed on original, continuation added for
  new element with correct position and duration
- Keyframes: classified by total per-keyframe duration
- Reverse iteration prevents stale animation ID collisions

Enhance splitElementInHtml:
- CSS rule duplication via PostCSS for ID-based styles
- Server-side ID deduplication for repeated splits
- Media playback-start adjustment for video/audio

Add split-animations route to gsap-mutations endpoint.

Author: miguel-heygen

bun.lock

@@ -193,6 +193,7 @@
     "@babel/parser": "^7.27.0",
     "@chenglou/pretext": "^0.0.5",
     "postcss": "^8.5.8",
+    "postcss-selector-parser": "^7.1.2",
     "recast": "^0.23.11"
   },
   "devDependencies": {