Descendant registry publishes state during effect commit #8606

Description

Primer's descendant registry registers and updates descendants from effects and publishes registry state synchronously from those effect paths. ActionList-heavy pages can therefore schedule registry updates during commit and produce cascading follow-up renders.

Expected behavior: descendant registration, value updates, cleanup, and ordering should be batched outside effect commit without stale entries or lost updates.

Steps to reproduce

1. Render an ActionList/ActionMenu tree with many descendants in a React 19 profiling build.
2. Mount, update, and unmount descendant items.
3. Record the commit/effects tracks.
4. Observe registry state publication originating from descendant effects.

Version

@primer/react 38.42.0

Browser

Chrome

Acceptance criteria

• Preserve stable ordering and descendant metadata.
• Cover rapid mount/unmount and update/unregister ordering.
• Avoid state publication directly from effect commit.
• Fix this in Primer React rather than requiring a downstream patch.