refactor(LayoutAnimations): stop taking over UIManagerAnimationDelegate

[bartlomiejbloniarz]

Summary

We register LayoutAnimationsProxy_Legacy as the UIManagerAnimationDelegate only to receive stopSurface — the other three delegate methods are no-ops. Occupying that slot overwrites the LayoutAnimationDriver that RN installs there, which silently kills LayoutAnimation.configureNext for the whole app.

The proxy now detects teardown itself: it registers a commit hook and marks a surface in surfacesToRemove_ whenever a commit's root has no children — which is what SurfaceHandler::stop → commitEmptyTree produces, and lands before that transaction reaches pullTransaction. The pull consumes the mark only on a transaction that actually removes children of the root, so pulls emitted for animation frames (which run with empty mutations) can't consume it early. App-level null renders don't trigger the mark (the AppContainer view stays under the root), so only real teardown and display-mode suspend commit an empty root — verified empirically and by enumerating every commitEmptyTree caller.

The check deliberately doesn't read the ShadowTreeRegistry: pullTransaction/commit hooks can run under the registry's shared lock, and re-locking there deadlocks with a pending writer (#8579 is that exact bug elsewhere).

Freeing the slot restores configureNext to stock RN behavior per platform: it animates on iOS (enableLayoutAnimationsOnIOS defaults to true) and on Android when the app enables enableLayoutAnimationsOnAndroid (off by default in RN core — without it configureNext snaps in stock RN too). The MountingCoordinator chains RN's driver and our override delegate; the driver's own stopSurface flushes its withheld removals into the teardown transaction, so the consume condition always holds there.

uiManager moves from the Android-only proxy constructor args to a shared one; the commit hook registration needs it on both platforms.

Test plan

fabric-example with ENABLE_SHARED_ELEMENT_TRANSITIONS=false (legacy proxy), iOS sim + Android emulator (with enableLayoutAnimationsOnAndroid force-enabled on Android):

• configureNext animates: video frame measurements show full easeInEaseOut interpolation on both platforms (iOS: 119 frames, 180→720px; Android: 158→275→388→468→514→630px)
• configureNext delete animations (transitions that remove views): the driver replays withheld Remove/Delete together with a final keyframe update; our reordering used to mount that update after the delete and abort (found in the Bluesky app, deterministic). Fixed in the second commit; covered by the bench's remove-views button on both platforms
• Reanimated entering/exiting/layout animations unchanged
• Reload teardown: exit animations skipped (single-frame removal), clean registry teardown, animations work on the fresh surface
• Adversarial pass: reloads fired at 0/60/150/350ms into exit animations, mount/unmount storms, concurrent reload requests, background/foreground with exits in flight, LogBox surface churn, configureNext mid-flight at teardown — 11/11 teardowns skipped exits with zero false positives on live surfaces, no crashes/ANRs/ghost views
• Rendering null at the app root: exits animate normally (surface alive), UI recovers

Manual test bench: remove-animation-delegate-testbench branch (flip ENABLE_SHARED_ELEMENT_TRANSITIONS to false in apps/fabric-example/package.json locally, then build).