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
