fix(studio): compute keyframe percentages in the tween's own time frame

A sub-composition tween's resolvedStart is composition-local, while the timeline
element resolved for it is the sub-comp HOST, whose start is main-timeline
absolute. toClipPercentage subtracted the two frames from each other, so a host
mounted at 1.5s cached its 0s tween at -12% and its last tween's end keyframe at
88% instead of 100%. A clip-relative percentage can never be negative.

resolveClipTimingBasis now returns the clip start in the frame the tween's own
times are measured in: the composition mount (expandedParentStart for an
expanded child, the parent composition clip's start otherwise, 0 for a
root-composition element) is subtracted, and a sub-comp inner element that falls
back to its host's window starts at 0 in that window. It moves to gsapShared so
the post-commit cache writer can share it instead of resolving its own basis,
which also gives that writer the sub-comp host fallback it was missing.

packages/studio/src/hooks/useGsapTweenCache.ts

@@ -7,24 +7,17 @@ import {
   pruneKeyframeCacheToFiles,
   writeGsapAnimationsForElement,
 } from "./gsapKeyframeCacheHelpers";
-import { toAbsoluteTime, toClipPercentage } from "./gsapShared";
+import { resolveClipTimingBasis, toAbsoluteTime, toClipPercentage } from "./gsapShared";
 import {
   deduplicateKeyframes,
   isStaticPositionHold,
   synthesizeFlatTweenKeyframes,
 } from "./gsapTweenSynth";
-import {
-  fetchParsedAnimations,
-  populateKeyframeCacheFromAst,
-  resolveClipTimingBasis,
-} from "./keyframeCacheAstLoad";
+import { fetchParsedAnimations, populateKeyframeCacheFromAst } from "./keyframeCacheAstLoad";
 
 // Re-exported so callers keep importing the GSAP cache surface from one module.
-export {
-  fetchParsedAnimations,
-  resolveClipTimingBasis,
-  resolveSelectorElementIds,
-} from "./keyframeCacheAstLoad";
+export { resolveClipTimingBasis } from "./gsapShared";
+export { fetchParsedAnimations, resolveSelectorElementIds } from "./keyframeCacheAstLoad";
 
 /** The selected element's identity for matching tweens to it. */
 export interface GsapElementTarget {