feat(persist): Suspense-based rehydration with React 19 use() hook

[ctrlplusb]

Summary

Modernize the persistence rehydration flow to leverage React 19's use() hook for Suspense integration, and wrap rehydration state replacement in startTransition for non-blocking hydration.

Parent issue: #1004

Background

useStoreRehydrated (hooks.js:81-90) currently uses the useState + useEffect pattern:

function useStoreRehydrated() {
  const [rehydrated, setRehydrated] = useState(false);
  useEffect(() => {
    store.persist.resolveRehydration().then(() => setRehydrated(true));
  }, []);
  return rehydrated;
}

This requires manual if (!rehydrated) checks in consumer code. React 19's use() hook enables Suspense integration instead.

Changes

1. Rewrite useStoreRehydrated using use()

• Replace useState + useEffect with use(store.persist.resolveRehydration())
• Enables <Suspense fallback={<Loading/>}> around rehydrating stores
• The current resolveRehydration() already returns a stable Promise (create-store.js:169)

2. Wrap rehydration replaceState in startTransition

• create-store.js:53 — the replaceState call during rehydration is a large state swap
• Wrapping in startTransition prevents it from blocking the UI

React version strategy

This requires a decision on React 19 support approach:

• Option A: Bump minimum peer dependency to React 19 (breaking change)
• Option B: Conditional export (easy-peasy/react19) with the use()-based hook
• Option C: Runtime feature detection (typeof React.use === 'function')

The startTransition portion is React 18 compatible and can land independently.

Testing

• Test Suspense boundary integration with useStoreRehydrated
• Test with async storage backends (the main rehydration path)
• Test startTransition wrapping doesn't break rehydration ordering
• Regression test existing useStoreRehydrated behavior

[ctrlplusb]

Reopening — the startTransition portion shipped in #1024 but the use()-based useStoreRehydrated migration is still outstanding. Now unblocked by the React 19 minimum-peer-dep bump (#1025).