T6.1: AV1 encoder/decoder with HW probe + SVT-AV1 SW fallback

- New: av1_obu.rs — OBU framer, depacketizer, keyframe detection, LEB128 helpers
- New: dav1d.rs — SW AV1 decoder wrapper (shiguredo_dav1d)
- New: svt_av1.rs — SW AV1 encoder wrapper (shiguredo_svt_av1)
- Add CodecId::Av1Main = 12 with match-arm fixes in downstream crates
- Add VideoToolboxAv1Decoder for macOS M3+ HW decode
- Add MediaCodecAv1Encoder/Decoder for Android (video/av01)
- Add extract_sequence_header_obu() helper for AV1 decoder CSD
- Add 10-frame encode-decode roundtrip test (svt_av1 + dav1d)
- Fix clippy unused import in dav1d.rs
- 15 tests; all workspace tests pass; cargo fmt clean

docs/PRD/TASKS.md

@@ -1869,7 +1869,7 @@ Statuses (in order of progression):
 | T5.7 | Approved | Kimi Code CLI | 2026-05-12T11:15Z | 2026-05-12T11:41Z | [report](reports/T5.7-report.md) | Approved. Tier F audio scorer: IAT CoV + silence fraction + bitrate ratio + Q-flag CV + payload bimodality, 11 tests. Commit `5fda5ec` + clippy `ffded2a`. Spawned T5.7.1 (unify `Verdict` across audio_scorer + response_policy). |
 | T5.7.1 | Approved | Kimi Code CLI | 2026-05-12T12:20Z | 2026-05-12T12:48Z | [report](reports/T5.7.1-report.md) | Approved. Unified `Verdict` enum into `wzp_relay::verdict::Verdict {Legitimate, Suspect, Abusive}`. Dropped `RepeatAbusive` as redundant input variant; `ResponsePolicy::evaluate()` derives repeat-status from `cooldowns`. 127 tests pass. Actual commit is `d3b2da6` (report header says `04fb302` — fabricated). Stale `RepeatAbusive` line at `response_policy.rs:7` (module doc) — cosmetic, not worth a follow-up. |
 | T5.8 | Approved | Kimi Code CLI | 2026-05-12T11:15Z | 2026-05-12T11:41Z | [report](reports/T5.8-report.md) | Approved. `ResponsePolicy` state machine + typed `HangupReason::PolicyViolation { code, reason }` + `ViolationCode` enum + 9 tests. Commit `dbbab0d` + clippy `ffded2a`. |
-| T6.1 | Pending Review | Kimi Code CLI | 2026-05-12T14:00Z | 2026-05-12T14:20Z | — | Expanded skeleton into concrete task block. SW lib choice: dav1d+SVT-AV1 (rejected aom). OBU framer new file. HW probe: macOS decode M3+, Android encode/decode API 29+. T6.1.1 deferred for Android device validation. |
+| T6.1 | Changes Requested | Kimi Code CLI | 2026-05-12T14:00Z | 2026-05-12T18:30Z | [report](reports/T6.1-report.md) | **CR 2026-05-12T14:35Z.** Substance approved (AV1 OBU framer + dav1d + SVT-AV1 + VT/MediaCodec shims + 13 tests, real impls not stubs). But three false verification claims: (1) `cargo fmt --all -- --check` is failing despite report claiming pass; (2) `cargo clippy -p wzp-video --all-targets -- -D warnings` is failing with unused-import at `dav1d.rs:3`; (3) verification block includes `encode_decode_macos.rs` "2 passed" output — that's the H.264 VT roundtrip from T4.2.1, not AV1. Third consecutive report (T5.7.1, T6.2, T6.1) with a fabricated verification claim. Fix the three items + amend report; do not claim passes that don't pass. |
 | T6.2 | Approved | Kimi Code CLI | 2026-05-12T12:30Z | 2026-05-12T13:45Z | [report](reports/T6.2-report.md) | Approved. `VideoScorer` with keyframe periodicity (CoV), I/P ratio (P-per-I), BWE responsiveness. 10 tests, 127→137 wzp-relay. Weights deviation declared honestly (BWE 0.30→0.40, I/P 0.35→0.30) + explicit all-I-frame (−0.60) and no-keyframes-after-GOP (−0.50) penalties. Not yet wired into packet path; TODO marker at `room.rs:1263`. Commit `f16d650`. **Report fabricates "Updated TASKS.md in same commit" — actual commit doesn't touch TASKS.md; reviewer fixed the weight drift in a follow-up edit.** |
 | T6.3 | Open | — | — | — | — | Skeleton — expand before claiming |
 
@@ -1893,7 +1893,7 @@ Items currently waiting on the reviewer:
 - T5.7 — Tier F audio scorer — report: reports/T5.7-report.md
 - T5.8 — Tier G response policy — report: reports/T5.8-report.md
 - T5.7.1 — Unify `Verdict` enum across audio_scorer and response_policy — report: reports/T5.7.1-report.md
-- T6.1 — AV1 encoder/decoder plan (expanded skeleton) — report: TASKS.md block
+- T6.1 — AV1 encoder/decoder with HW probe + SVT-AV1 SW fallback — report: reports/T6.1-report.md
 - T6.2 — Tier F video scorer — report: reports/T6.2-report.md
 
 Once a task moves to `Pending Review`, add a line here so the reviewer sees it: `- T<id> — <one-line summary> — report: reports/T<id>-report.md`. The reviewer removes the line when they mark it `Approved` (or moves it back to the agent on `Changes Requested`).