test(01-02): add RED ring-buffer tests

Four RED tests pin D-10 (header pinning) and D-11 (30s trim) contracts:
- 'first chunk is header' — isFirst marker on first addChunk
- 'second chunk is NOT header' — only the first is pinned
- 'trim 30s — keeps header, evicts aged tail' — header survives indefinitely
- 'trim with empty buffer does not throw' — defensive edge case

Plan 03 must export {addChunk, trimAged, getBuffer, resetBuffer} from
src/offscreen/recorder.ts to flip these to GREEN.

Also stages tests/fixtures/.gitkeep so the fixture dir survives clean
checkouts (Plan 07 drops a known-good last_30sec.webm into it after the
manual smoke test).

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

tests/offscreen/ring-buffer.test.ts

@@ -0,0 +1,40 @@
+import { describe, it, expect, beforeEach } from 'vitest';
+import { addChunk, trimAged, getBuffer, resetBuffer } from '../../src/offscreen/recorder';
+
+describe('ring buffer', () => {
+  beforeEach(() => resetBuffer());
+
+  it('first chunk is header', () => {
+    addChunk({ size: 1024 } as unknown as Blob, 1_000);
+    const buf = getBuffer();
+    expect(buf.length).toBe(1);
+    expect(buf[0].isFirst).toBe(true);
+  });
+
+  it('second chunk is NOT header', () => {
+    addChunk({ size: 1024 } as unknown as Blob, 1_000);
+    addChunk({ size: 512 } as unknown as Blob, 2_000);
+    const buf = getBuffer();
+    expect(buf.length).toBe(2);
+    expect(buf[0].isFirst).toBe(true);
+    expect(buf[1].isFirst).toBeFalsy();
+  });
+
+  it('trim 30s — keeps header, evicts aged tail', () => {
+    addChunk({ size: 1024 } as unknown as Blob, 0);          // header at t=0
+    addChunk({ size: 512 } as unknown as Blob, 10_000);      // t=10s
+    addChunk({ size: 512 } as unknown as Blob, 35_000);      // t=35s
+    trimAged(40_000);                                        // now=40s
+    const buf = getBuffer();
+    expect(buf[0].isFirst).toBe(true);                       // header survives unconditionally
+    expect(buf.length).toBeGreaterThanOrEqual(2);            // header + at least the t=35s chunk
+    // The header chunk's age (40s) does NOT cause it to be trimmed.
+    const headerStillThere = buf.some((c) => c.isFirst);
+    expect(headerStillThere).toBe(true);
+  });
+
+  it('trim with empty buffer does not throw', () => {
+    expect(() => trimAged(0)).not.toThrow();
+    expect(getBuffer()).toEqual([]);
+  });
+});