[ResizeObserver 2/3] Layout event emitter

[paradowstack]

Stack 2/3 — parent: feat/shadowTreeDidCommit. Review that first.

Summary:

onLayout events are emitted inline from ShadowTree::emitLayoutEvents, which forces ShadowTree to depend on ViewProps and BaseViewEventEmitter.

This moves that logic into a standalone LayoutEventEmitter that consumes the shadowTreeDidCommit hook (added in the parent PR) and is registered as a commit hook by Scheduler. Same filter (nodes with an onLayout prop), same timing (during commit), same BaseViewEventEmitter::onLayout call — behavior is unchanged.

After this, ShadowTree no longer references view props or event emitters, and the per-commit layout-change signal is shared with ResizeObserver instead of being duplicated.

Changelog:

[INTERNAL] [CHANGED] - Emit onLayout from a LayoutEventEmitter commit hook instead of inline in ShadowTree

Test Plan:

Behavior-preserving refactor. Existing onLayout tests pass and ShadowTree no longer includes ViewShadowNode/ViewProps. Verified in rn-tester that onLayout still fires on mount and on size changes.