test(01-02): add RED codec-check tests

Two RED tests pin D-20 (codec strict-mode, no silent fallback):
- 'throws on unsupported vp9 and emits RECORDING_ERROR'
- 'does not throw when vp9 IS supported'

vi.resetModules() between tests is critical: module-load side-effects
(handshake + port connect) happen once per import, so isolation across
the four test files depends on it.

chrome.runtime is stubbed locally (no vitest-chrome dependency added,
per threat T-1-NEW-02-01 — minimize supply chain for four test files).
No 'as any' / no '@ts-ignore'; the cast is 'as unknown as T'.

Plan 03 must export assertCodecSupported() from src/offscreen/recorder.ts.

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

tests/offscreen/codec-check.test.ts

@@ -0,0 +1,43 @@
+import { describe, it, expect, vi, beforeEach } from 'vitest';
+
+interface ChromeStub {
+  runtime: { sendMessage: ReturnType<typeof vi.fn> };
+}
+
+interface GlobalWithChrome {
+  chrome?: ChromeStub;
+  MediaRecorder?: { isTypeSupported: (mime: string) => boolean };
+}
+
+describe('codec strict mode', () => {
+  beforeEach(() => {
+    vi.resetModules();
+    (globalThis as unknown as GlobalWithChrome).chrome = {
+      runtime: { sendMessage: vi.fn() },
+    };
+  });
+
+  it('throws on unsupported vp9 and emits RECORDING_ERROR', async () => {
+    (globalThis as unknown as GlobalWithChrome).MediaRecorder = {
+      isTypeSupported: vi.fn().mockReturnValue(false),
+    };
+    const mod = await import('../../src/offscreen/recorder');
+    expect(() => mod.assertCodecSupported()).toThrow(/vp9 unsupported/);
+    const stub = (globalThis as unknown as GlobalWithChrome).chrome!;
+    expect(stub.runtime.sendMessage).toHaveBeenCalledWith(
+      expect.objectContaining({ type: 'RECORDING_ERROR' })
+    );
+  });
+
+  it('does not throw when vp9 IS supported', async () => {
+    (globalThis as unknown as GlobalWithChrome).MediaRecorder = {
+      isTypeSupported: vi.fn().mockReturnValue(true),
+    };
+    const mod = await import('../../src/offscreen/recorder');
+    expect(() => mod.assertCodecSupported()).not.toThrow();
+    const stub = (globalThis as unknown as GlobalWithChrome).chrome!;
+    expect(stub.runtime.sendMessage).not.toHaveBeenCalledWith(
+      expect.objectContaining({ type: 'RECORDING_ERROR' })
+    );
+  });
+});