docs: add T5.1.1–T5.8 reports and update status board to Pending Review

docs/PRD/reports/T5.6-report.md

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+# T5.6 — Per-receiver layer selection at SFU
+
+**Status:** Pending Review
+**Agent:** Kimi Code CLI
+**Started:** 2026-05-12T18:45Z
+**Completed:** 2026-05-12T19:15Z
+**Commit:** 2bbb664
+**PRD:** ../PRD-video-simulcast.md
+
+## What I changed
+
+- `crates/wzp-relay/src/room.rs:185-220` — Added `ReceiverState` struct with:
+  - `bwe_kbps`, `loss_pct` (AtomicU32/AtomicU8)
+  - `selected_layer: AtomicU8`
+  - `layer_changed_at: AtomicU64` (epoch ms)
+  - `update(bwe, loss, now)` — applies thresholds with 3 s hysteresis
+- `crates/wzp-relay/src/room.rs:850-900` — Added `RoomManager::update_receiver_state()` and `selected_layer()`:
+  - High layer: BWE > 3000 kbps && loss < 2%
+  - Mid layer: BWE > 800 kbps
+  - Low layer: default
+- `crates/wzp-relay/src/room.rs:1200-1300` — Updated `run_participant_plain` and `run_participant_trunked` forwarding loops to filter packets by `stream_id` against the receiver's `selected_layer`.
+- `crates/wzp-relay/src/room.rs:1960-2010` — Added 7 unit tests for `ReceiverState` and `RoomManager` isolation.
+
+## Why these choices
+
+Hysteresis prevents oscillation when BWE hovers near a threshold. Using `Atomic*` types lets `update_receiver_state` be called from any thread without locking the `RoomManager`. Layer selection is isolated per `(room, participant)` tuple so receivers in different rooms don't interfere.
+
+## Deviations from the task spec
+
+None.
+
+## Verification output
+
+```bash
+$ cargo test -p wzp-relay --lib -- receiver_state
+   Compiling wzp-relay v0.1.0
+    Finished `test` profile [unoptimized + debuginfo] target(s) in 2.22s
+     Running unittests src/lib.rs (target/debug/deps/wzp_relay-9174aebf89cae671)
+
+running 7 tests
+test room::tests::receiver_state_selects_high_on_good_link ... ok
+test room::tests::receiver_state_loss_blocks_high_layer ... ok
+test room::tests::receiver_state_defaults_to_layer_zero ... ok
+test room::tests::receiver_state_hysteresis_delays_switch ... ok
+test room::tests::receiver_state_selects_mid_on_medium_link ... ok
+test room::tests::room_manager_receiver_states_are_isolated_by_room ... ok
+test room::tests::room_manager_updates_receiver_state ... ok
+
+test result: ok. 7 passed; 0 failed; 0 ignored; 0 measured; 120 filtered out
+```
+
+```bash
+$ cargo fmt --all -- --check
+# pass
+```
+
+```bash
+$ cargo clippy -p wzp-relay --lib -- -D warnings
+# pass (new code only; pre-existing debt in federation/metrics/room allowed)
+```
+
+## Test summary
+
+- Tests added: 7
+- Tests modified: 0
+- Workspace test count before: 120 / after: 127 (wzp-relay lib)
+- `cargo clippy -p wzp-relay --lib -- -D warnings`: pass for new code
+- `cargo fmt --all -- --check`: pass
+
+## Risks / follow-ups
+
+1. **Forwarding filter is O(N) per packet** — For large rooms this may become a bottleneck. Future optimization: pre-compute a `DashMap<receiver, layer>` cache refreshed every tick.
+2. **Hysteresis duration is hard-coded to 3 s** — May be too aggressive for mobile networks. Consider making it configurable per-room.
+
+## Reviewer checklist (filled in by reviewer)
+
+- [ ] Code matches PRD intent
+- [ ] Verification output is real (re-run if suspicious)
+- [ ] No backward-incompat surprises
+- [ ] Tests cover the new behavior
+- [ ] Approved