refactor(studio): extract shared timeline components and deduplicate code (#1329)

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

packages/studio/src/utils/timelineElementSplit.ts

@@ -0,0 +1,13 @@
+import type { TimelineElement } from "../player/store/playerStore";
+
+export { buildPatchTarget, readFileContent } from "../hooks/timelineEditingHelpers";
+
+export function canSplitElement(el: TimelineElement): boolean {
+  return (
+    !el.timelineLocked &&
+    el.timingSource !== "implicit" &&
+    !el.compositionSrc &&
+    !!el.duration &&
+    Number.isFinite(el.duration)
+  );
+}