feat(react): add useWrnStore bridge over useSyncExternalStore

Resolves WRNexus stores from inside islands, reading through the cached
snapshot so React sees a stable reference. Unknown store names throw
with the list of registered stores rather than failing opaquely.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

packages/react/src/index.ts

@@ -0,0 +1,4 @@
+export { createSelectorCache, createSnapshotCache } from "./snapshot-cache.ts";
+export type { SnapshotCache, SnapshotSource } from "./snapshot-cache.ts";
+export { setStoreResolver, useWrnActions, useWrnStore } from "./store-bridge.ts";
+export type { BoundStore, IslandStore, StoreResolver } from "./store-bridge.ts";