test(01-02): add RED handshake + port tests

Three RED tests pin Pattern 4 (handshake) and Pattern 5 / Pitfall 4
(port reconnect on disconnect) contracts:

handshake.test.ts:
- 'sends OFFSCREEN_READY after listener registration' — exactly one
  OFFSCREEN_READY emitted at module load, AFTER onMessage.addListener

port.test.ts:
- 'connects on module load' — chrome.runtime.connect called once
- 'reconnects when port disconnects' — firing onDisconnect triggers
  immediate re-connect (Pitfall 4 idle-timer reset)

chrome.runtime is stubbed locally (no vitest-chrome dependency added).
No 'as any' / no '@ts-ignore'; casts are 'as unknown as T'.

Plan 04 must wire OFFSCREEN_READY send + port.connect({ name:
'video-keepalive' }) + onDisconnect-driven reconnect at the import-side
effect layer of src/offscreen/recorder.ts.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

tests/offscreen/handshake.test.ts

@@ -0,0 +1,67 @@
+import { describe, it, expect, vi, beforeEach } from 'vitest';
+
+interface PortStub {
+  name: string;
+  postMessage: ReturnType<typeof vi.fn>;
+  onMessage: { addListener: ReturnType<typeof vi.fn> };
+  onDisconnect: { addListener: ReturnType<typeof vi.fn> };
+  disconnect: ReturnType<typeof vi.fn>;
+}
+
+interface ChromeStub {
+  runtime: {
+    id: string;
+    sendMessage: (m: unknown) => void;
+    onMessage: { addListener: ReturnType<typeof vi.fn> };
+    connect: () => PortStub;
+  };
+}
+
+interface GlobalWithChrome {
+  chrome?: ChromeStub;
+  MediaRecorder?: { isTypeSupported: (mime: string) => boolean };
+}
+
+function buildChromeStub(calls: unknown[]): ChromeStub {
+  return {
+    runtime: {
+      id: 'ext-id-test',
+      sendMessage: (m: unknown) => {
+        calls.push(m);
+      },
+      onMessage: { addListener: vi.fn() },
+      connect: () => ({
+        name: 'video-keepalive',
+        postMessage: vi.fn(),
+        onMessage: { addListener: vi.fn() },
+        onDisconnect: { addListener: vi.fn() },
+        disconnect: vi.fn(),
+      }),
+    },
+  };
+}
+
+describe('OFFSCREEN_READY handshake', () => {
+  beforeEach(() => {
+    vi.resetModules();
+    (globalThis as unknown as GlobalWithChrome).MediaRecorder = {
+      isTypeSupported: vi.fn().mockReturnValue(true),
+    };
+  });
+
+  it('sends OFFSCREEN_READY after listener registration', async () => {
+    const calls: unknown[] = [];
+    const stub = buildChromeStub(calls);
+    (globalThis as unknown as GlobalWithChrome).chrome = stub;
+    await import('../../src/offscreen/recorder');
+    expect(stub.runtime.onMessage.addListener).toHaveBeenCalled();
+    expect(calls).toEqual(
+      expect.arrayContaining([expect.objectContaining({ type: 'OFFSCREEN_READY' })])
+    );
+    const readyCount = calls.filter(
+      (m): m is { type: string } =>
+        typeof m === 'object' && m !== null && (m as { type?: unknown }).type === 'OFFSCREEN_READY'
+    ).length;
+    expect(readyCount).toBe(1);
+  });
+});