feat(fix-a3): retire ring-buffer first-chunk pin tests, add segment-rotation contract

Per debug session webm-playback-freeze "Activation Plan" step 4: the
D-09..D-11 ring-buffer semantics (first-chunk header pin + 30 s age trim)
are being replaced by D-13 restart-segments. The pinned-header assertions
were architecture-specific and become meaningless once each segment is
a self-contained WebM with its own EBML header and seed keyframe.

Changes:
- tests/offscreen/ring-buffer.test.ts: collapsed to a single breadcrumb
  test pointing at the successor file. Kept the path so git history /
  failure bisects land on the retirement commit cleanly.
- tests/offscreen/segment-rotation.test.ts (new): 8 tests pinning the
  D-13 invariants against the production recorder module:
    * MAX_SEGMENTS = 3, SEGMENT_DURATION_MS = 10_000 (= legacy 30 s window)
    * empty-by-default, ordered, oldest-evicted-at-cap
    * resetBuffer clears
    * getSegments returns a defensive snapshot (no internal aliasing)
  Uses a `pushSegmentForTest` seam so vitest can drive rotation
  deterministically without instantiating a real MediaRecorder.

RED today by design (TDD discipline) — the segment-rotation suite
imports `getSegments`, `pushSegmentForTest`, `MAX_SEGMENTS`,
`SEGMENT_DURATION_MS` from src/offscreen/recorder.ts. Those exports
land in the next commit. tsconfig.include is "src" only so tsc stays
clean during the RED window.

tests/offscreen/ring-buffer.test.ts

@@ -1,40 +1,29 @@
-import { describe, it, expect, beforeEach } from 'vitest';
-import { addChunk, trimAged, getBuffer, resetBuffer } from '../../src/offscreen/recorder';
+// tests/offscreen/ring-buffer.test.ts — vestigial entry point.
+//
+// Under D-09..D-11 this file pinned the single-continuous-recorder ring
+// buffer (first-chunk header pin + 30 s age-trim). Those semantics are
+// retired in favour of the D-13 restart-segments architecture (debug
+// session webm-playback-freeze): each segment is a self-contained ~10 s
+// WebM with its own EBML header and seed keyframe, so a special
+// `isFirst` flag is no longer meaningful.
+//
+// The equivalent invariants under D-13 are exercised in
+// `tests/offscreen/segment-rotation.test.ts`. This file is kept as a
+// breadcrumb so a future reader grepping for "ring buffer" lands on the
+// correct successor — and so the retirement is auditable in `git log`
+// (the original 4 tests were deleted in the same commit that added the
+// segment-rotation suite).
+//
+// See: .planning/debug/webm-playback-freeze.md "Activation Plan"
+//      step 4 — the D-09..D-11 invariants are replaced, not weakened.
 
-describe('ring buffer', () => {
-  beforeEach(() => resetBuffer());
+import { describe, it, expect } from 'vitest';
 
-  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([]);
+describe('ring buffer (retired — see segment-rotation.test.ts)', () => {
+  it('D-09..D-11 first-chunk pin is retired by D-13 restart-segments', () => {
+    // Structural assertion only: the test exists to make the retirement
+    // visible in the test report. Behavioural assertions live in
+    // segment-rotation.test.ts.
+    expect(true).toBe(true);
   });
 });