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/player/components/timelineCallbacks.ts

@@ -0,0 +1,44 @@
+// fallow-ignore-file code-duplication
+// fallow-ignore-file dead-code
+import type { TimelineElement } from "../store/playerStore";
+import type { BlockedTimelineEditIntent } from "./timelineEditing";
+
+/**
+ * Shared callback signatures for timeline editing operations.
+ * Used by NLELayout, Timeline, and any component that passes through
+ * the standard set of timeline mutation handlers.
+ */
+export interface TimelineDropCallbacks {
+  onFileDrop?: (
+    files: File[],
+    placement?: { start: number; track: number },
+  ) => Promise<void> | void;
+  onAssetDrop?: (
+    assetPath: string,
+    placement: { start: number; track: number },
+  ) => Promise<void> | void;
+  onBlockDrop?: (
+    blockName: string,
+    placement: { start: number; track: number },
+  ) => Promise<void> | void;
+}
+
+export interface TimelineEditCallbacks {
+  onMoveElement?: (
+    element: TimelineElement,
+    updates: Pick<TimelineElement, "start" | "track">,
+  ) => Promise<void> | void;
+  onResizeElement?: (
+    element: TimelineElement,
+    updates: Pick<TimelineElement, "start" | "duration" | "playbackStart">,
+  ) => Promise<void> | void;
+  onBlockedEditAttempt?: (element: TimelineElement, intent: BlockedTimelineEditIntent) => void;
+  onSplitElement?: (element: TimelineElement, splitTime: number) => Promise<void> | void;
+  onRazorSplit?: (element: TimelineElement, splitTime: number) => Promise<void> | void;
+  onRazorSplitAll?: (splitTime: number) => Promise<void> | void;
+  onDeleteKeyframe?: (elementId: string, percentage: number) => void;
+  onDeleteAllKeyframes?: (elementId: string) => void;
+  onChangeKeyframeEase?: (elementId: string, percentage: number, ease: string) => void;
+  onMoveKeyframe?: (element: TimelineElement, oldPct: number, newPct: number) => void;
+  onToggleKeyframeAtPlayhead?: (element: TimelineElement) => void;
+}