dillon1000/react

Add eager alternate.stateNode cleanup (#33161)

This is a fix for a problem where React retains shadow nodes longer than
it needs to. The behaviour is shown in React Native test:
https://github.com/facebook/react-native/blob/main/packages/react-native/src/private/__tests__/utilities/__tests__/ShadowNodeReferenceCounter-itest.js#L169

# Problem
When React commits a new shadow tree, old shadow nodes are stored inside
`fiber.alternate.stateNode`. This is not cleared up until React clones
the node again. This may be problematic if mutation deletes a subtree,
in that case `fiber.alternate.stateNode` will retain entire subtree
until next update. In case of image nodes, this means retaining entire
images.

So when React goes from revision A: `<View><View /></View>` to revision
B: `<View />`, `fiber.alternate.stateNode` will be pointing to Shadow
Node that represents revision A..


![image](https://github.com/user-attachments/assets/076b677e-d152-4763-8c9d-4f923212b424)


# Fix
To fix this, this PR adds a new feature flag
`enableEagerAlternateStateNodeCleanup`. When enabled,
`alternate.stateNode` is proactively pointed towards finishedWork's
stateNode, releasing resources sooner.

I have verified this fixes the issue [demonstrated by React Native
tests](https://github.com/facebook/react-native/blob/main/packages/react-native/src/private/__tests__/utilities/__tests__/ShadowNodeReferenceCounter-itest.js#L169).
All existing React tests pass when the flag is enabled.