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/test/store-bridge.test.ts

@@ -0,0 +1,50 @@
+import { expect, test } from "bun:test";
+import { getStoreForTest, setStoreResolver } from "../src/store-bridge.ts";
+
+function fakeStore(initial: { count: number }) {
+  let state = { ...initial };
+  const listeners = new Set<() => void>();
+  return {
+    snapshot: () => Object.freeze({ ...state }),
+    subscribe(listener: () => void) {
+      listeners.add(listener);
+      return () => listeners.delete(listener);
+    },
+    actions: {
+      increment: () => {
+        state = { count: state.count + 1 };
+        for (const listener of [...listeners]) listener();
+      },
+    },
+  };
+}
+
+test("resolves a registered store and caches its snapshot", () => {
+  const store = fakeStore({ count: 0 });
+  setStoreResolver((name) => (name === "counter" ? store : undefined), ["counter"]);
+
+  const bound = getStoreForTest<{ count: number }>("counter");
+  expect(bound.getSnapshot()).toBe(bound.getSnapshot());
+
+  bound.store.actions.increment!();
+  expect(bound.getSnapshot().count).toBe(1);
+
+  setStoreResolver(null);
+});
+
+test("throws a helpful error for an unknown store name", () => {
+  setStoreResolver(
+    (name) => (name === "counter" ? fakeStore({ count: 0 }) : undefined),
+    ["counter"],
+  );
+
+  expect(() => getStoreForTest("typo")).toThrow(/Unknown WRNexus store "typo"/);
+  expect(() => getStoreForTest("typo")).toThrow(/counter/);
+
+  setStoreResolver(null);
+});
+
+test("throws when no resolver has been registered yet", () => {
+  setStoreResolver(null);
+  expect(() => getStoreForTest("counter")).toThrow(/before the island runtime registered/);
+});