refactor(studio): tighten the expanded-lane cache and height helpers

Validate the parse response before reading `.animations` instead of casting
the JSON blind, and narrow the fetch's return type to the slice callers read.
Route the AST cache load through the shared clip-keyframe and cache-key
helpers so it can't drift from the other writer. Drop the unused numeric
track-count branches from `trackHeights`/`getTimelineCanvasHeight`, and pick
the widest keyframed clip with a reduce so there's no index assertion.

packages/studio/src/hooks/useGsapAnimationFetchFallback.ts

@@ -36,7 +36,7 @@ export type ElementAnimationsOutcome =
  * so the caller can apply the right retry budget to each.
  */
 export function selectElementAnimationsOrRetry(
-  parsed: ParsedGsap | null,
+  parsed: Pick<ParsedGsap, "animations"> | null,
   target: { id: string | null; selector: string | null },
 ): ElementAnimationsOutcome {
   if (!parsed) return { kind: "fetch-error" };