feat(dom): add useThrottledCallback hook + fix useSyncExternalStore stale closure

[srpatcha]

Summary

Two React improvements:

1. useThrottledCallback hook (new) — packages/react-dom/src/hooks/useThrottledCallback.js

   • Configurable delay with leading/trailing edge options
   • cancel() and flush() API for pending invocations
   • Pending-state tracking
   • Proper cleanup on unmount (timer cancellation)
   • Uses refs to always invoke latest callback without re-creating throttled fn

2. useSyncExternalStore stale-closure fix — packages/react-reconciler/src/ReactFiberHooks.js

   • Adds isSubscribed guard to handleStoreChange to prevent the callback from firing after the component has conceptually unmounted but before the passive-effect cleanup runs
   • Cleanup sets isSubscribed=false before calling unsubscribe, closing the timing window where handleStoreChange could operate on a stale fiber reference

3. Jest tests — packages/react-dom/src/__tests__/useThrottledCallback-test.js (331 lines)

   • Leading edge immediate invocation
   • Throttling within delay window
   • Leading-only and trailing-only modes
   • cancel() prevents pending trailing invocations
   • flush() triggers immediate invocation
   • Latest callback ref is used without re-creating throttled fn
   • Timer cleanup on unmount prevents stale invocations

Scope

Was previously titled "fix: handle null-prototype objects in typeName helper" — that commit's actual code change was lost during an earlier squash, leaving only file-permission changes that conflicted with upstream deletions. Dropped the empty-content commit and re-scoped this PR to the 3 substantive hooks/test changes.

Files

File	Δ
packages/react-dom/src/hooks/useThrottledCallback.js	+228 / 0 (new)
packages/react-dom/src/__tests__/useThrottledCallback-test.js	+331 / 0 (new)
packages/react-reconciler/src/ReactFiberHooks.js	+14 / -1
Total	+573 / -1

[srpatcha]

Cleaned up + rebased onto current main (commit 171ed8c):

• ✅ Rebased onto current main (was BEHIND with merge conflict)
• ✅ Dropped the empty-content a8dd8395 commit (its actual typeName code was lost in an earlier squash, leaving only file-mode changes that conflicted with upstream's release-script deletions)
• ✅ Re-scoped PR to the 3 substantive commits: useThrottledCallback hook + useSyncExternalStore stale-closure fix + Jest tests
• ✅ Updated title and body to reflect actual scope

Ready for re-review!