…g render

Move node registration from useEffect to the render phase so that
useSyncExternalStore's getSnapshot returns the correct hasFocus value
on the first render. This eliminates the visible one-frame lag when
opening collapsible sections or returning to router screens.

- Register nodes silently during render (no subscriber notifications)
- Flush deferred notifications in useLayoutEffect before paint
- Extract passive-clearing into a shared helper for both silent and
  normal focus transitions
- Track focusKey on FocusNode to handle key changes without destroying
  the node's children
- Re-register in useLayoutEffect setup for React Strict Mode compat