feat(01-07): close Phase 1 — REQ-video-ring-buffer complete, SPEC §10 #7 satisfied

Phase 1 closure 2026-05-15 — both acceptance gates green:

- D-12 ffprobe structural gate: `ffprobe -v error -f matroska -i
  tests/fixtures/last_30sec.webm` exit 0 (cd61cbc)
- A3 empirical-playback gate: operator confirmed clean end-to-end Chrome
  playback (no ~1 s freeze); ffmpeg `-v warning -i fixture -f null -`
  exit 0 with zero decoder errors (only expected muxer DTS-monotonicity
  warnings at segment join boundaries — documented D-13 trade-off for
  multi-EBML-header WebM concat; Chrome's MSE pipeline handles this
  natively, satisfying SPEC §10 #7)

Changes:

- .planning/REQUIREMENTS.md
  * REQ-video-ring-buffer checkbox [ ] -> [x]; description AMENDED to
    document the D-13 restart-segments lifecycle replacing D-09..D-11;
    SPEC §10 #2, #3, #7 noted as green 2026-05-15
  * Traceability table row: REQ-video-ring-buffer | Phase 1 | Complete
    (was Pending)

- .planning/ROADMAP.md
  * Phase 1 list-item flipped [ ] -> [x] with closure date + summary
  * Phase 1 Success Criteria 1, 2, 3 individually checked off; criterion 2
    re-worded to reflect D-13 segment-cycling (replacing the original
    single-continuous-recorder wording from D-09..D-11)
  * Plan list: 01-07-PLAN.md flipped [ ] -> [x] with closure note
  * Progress table row: 7/7 Complete 2026-05-15 (was 6/7 In Progress)
  * Phase 5 P1/P2 list: appended `getDisplayMedia` cursor visibility
    constraint (`video: { cursor: 'always' }`) — surfaced as user
    observation during Phase 1 smoke 2026-05-15

- .planning/STATE.md
  * Frontmatter: status -> phase_complete, completed_phases 0 -> 1,
    completed_plans 6 -> 7, percent 86 -> 100; stopped_at + last_activity
    rewritten for closure narrative
  * Current Position: Phase 1 COMPLETE, next Phase 2 of 5, Plan 7/7
    complete, progress bar [██████████] 100% Phase 1
  * Performance Metrics: Phase 1 row populated (7 plans, ~50 min); added
    Plan 07 row with closure narrative incl. the two debug sessions
  * Decisions log: appended [Phase 01-07-closure] decision and
    [Phase 01-07-deferred-to-5] note for the cursor-visibility refinement
  * Session Continuity: rewritten for closure; resume file points to the
    Plan 07 SUMMARY (commit 3)
  * Added "Phase 1 Closure Notes" block with ffprobe + ffmpeg gates,
    fixture metadata, test counts, criteria green-status, and a process
    retro candidate (auto-injection of empirical-acceptance gates when
    RESEARCH.md flags HIGH-risk assumptions)

Refs: .planning/debug/resolved/d12-blob-port-transfer-fails.md,
.planning/debug/resolved/webm-playback-freeze.md

.planning/REQUIREMENTS.md

@@ -16,21 +16,28 @@ Requirements for the Phase 1 SPEC. Each maps to one phase in ROADMAP.md.
 
 ### Video
 
-- [ ] **REQ-video-ring-buffer**: The extension maintains an in-memory ring
+- [x] **REQ-video-ring-buffer**: The extension maintains an in-memory ring
   buffer containing the most recent 30 seconds of captured video. AMENDED in
   Phase 01: video is acquired via `navigator.mediaDevices.getDisplayMedia()`
   invoked from the offscreen document (with `chrome.offscreen.Reason.DISPLAY_MEDIA`),
   NOT `chrome.tabCapture` as originally specified. The captured stream is
   screen-or-window-scoped per the operator's one-time selection in Chrome's
   native picker, and continues unchanged across tab switches. Encoding is
-  unchanged: `video/webm; codecs=vp9` @ 400 000 bps with a `MediaRecorder`
-  timeslice of 2000 ms; a single continuous recorder runs for the whole
-  session. The first emitted chunk (WebM header) is retained indefinitely;
-  subsequent chunks rotate out by the 30-second TTL rule. Bindings: DEC-003
-  (AMENDED), DEC-009, CON-video-window, CON-video-codec,
-  CON-webm-header-retention, CON-display-capture-binding (replaces RETIRED
-  CON-tab-capture-binding).
-  - SPEC §10 acceptance criteria: #2, #3, #7.
+  `video/webm; codecs=vp9` @ 400 000 bps. Ring-buffer mechanism FURTHER
+  AMENDED in Phase 01 fix-a3 (debug session webm-playback-freeze, resolved
+  2026-05-15): the original D-09..D-11 single-continuous-`MediaRecorder` +
+  age-trim approach was retired in favor of D-13 restart-segments — the
+  recorder stop()/start()s every 10 s on the same `MediaStream`, keeping
+  the last 3 self-contained ~10 s WebM segments (3 × 10 s = 30 s window).
+  Each segment carries its own EBML header + seed VP9 keyframe and is
+  independently decodable, eliminating the orphan-P-frame freeze observed
+  with the trim approach. Bindings: DEC-003 (AMENDED), DEC-009,
+  CON-video-window, CON-video-codec, CON-display-capture-binding (replaces
+  RETIRED CON-tab-capture-binding). CON-webm-header-retention RETIRED in
+  favor of D-13 per-segment header isolation.
+  - SPEC §10 acceptance criteria: #2, #3, #7 — all green 2026-05-15
+    (D-12 ffprobe gate + operator-confirmed Chrome playback + ffmpeg dry-run
+    exit 0 with zero decoder errors against `tests/fixtures/last_30sec.webm`).
 
 ### DOM Capture
 
@@ -186,7 +193,7 @@ Which phase covers which requirement. See ROADMAP.md for phase details.
 
 | Requirement | Phase | Status |
 |-------------|-------|--------|
-| REQ-video-ring-buffer | Phase 1 | In Progress |
+| REQ-video-ring-buffer | Phase 1 | Complete |
 | REQ-rrweb-dom-buffer | Phase 2 | Pending |
 | REQ-user-event-log | Phase 2 | Pending |
 | REQ-password-confidentiality | Phase 2 | Pending |
@@ -210,4 +217,4 @@ RAM-ceiling check.
 
 ---
 *Requirements defined: 2026-05-15*
-*Last updated: 2026-05-15 after initial bootstrap from intel synthesis*
+*Last updated: 2026-05-15 — REQ-video-ring-buffer marked Complete on Phase 1 closure*