feat: add MessagingProvider and useAdapter

packages/iios-messaging-ui/src/provider.test.tsx

@@ -0,0 +1,25 @@
+import { describe, it, expect } from 'vitest';
+import { render, screen } from '@testing-library/react';
+import { MessagingProvider, useAdapter } from './provider';
+import { MockAdapter } from './adapters/mock';
+
+function ShowActor() {
+  const adapter = useAdapter();
+  return <span>{adapter.currentActorId()}</span>;
+}
+
+describe('MessagingProvider', () => {
+  it('supplies the injected adapter to descendants', () => {
+    render(
+      <MessagingProvider adapter={new MockAdapter()}>
+        <ShowActor />
+      </MessagingProvider>,
+    );
+    expect(screen.getByText('me')).toBeDefined();
+  });
+
+  it('throws a helpful error when a hook is used outside the provider', () => {
+    // React logs the error boundary trace; that noise is expected.
+    expect(() => render(<ShowActor />)).toThrow(/useAdapter must be used within a <MessagingProvider>/);
+  });
+});

packages/iios-messaging-ui/src/provider.tsx

@@ -0,0 +1,22 @@
+import { createContext, useContext, type ReactNode } from 'react';
+import type { MessagingAdapter } from './adapter';
+
+const AdapterContext = createContext<MessagingAdapter | null>(null);
+
+export function MessagingProvider({
+  adapter,
+  children,
+}: {
+  adapter: MessagingAdapter;
+  children: ReactNode;
+}) {
+  return <AdapterContext.Provider value={adapter}>{children}</AdapterContext.Provider>;
+}
+
+/** Access the host-injected adapter. Throws outside a provider — a missing provider is
+ *  a wiring bug, and failing loudly beats a confusing null-deref three layers down. */
+export function useAdapter(): MessagingAdapter {
+  const adapter = useContext(AdapterContext);
+  if (!adapter) throw new Error('useAdapter must be used within a <MessagingProvider>');
+  return adapter;
+}