T2.3-T2.6: BWE guard, relay conformance Tier A/B/C, Prometheus metrics

docs/PRD/reports/T2.3-report.md

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+# T2.3 — Plumb BWE into `AdaptiveQualityController`
+
+**Status:** Pending Review
+**Agent:** Kimi Code CLI
+**Started:** 2026-05-11T17:13Z
+**Completed:** 2026-05-11T17:20Z
+**Commit:** 846c98e
+**PRD:** ../PRD-transport-feedback-bwe.md
+
+## What I changed
+
+- `crates/wzp-proto/src/quality.rs` — `AdaptiveQualityController`:
+  - Added `bwe: Option<Arc<BandwidthEstimator>>` field.
+  - Added `set_bandwidth_estimator(&mut self, bwe: Arc<BandwidthEstimator>)` setter.
+  - Added `tier_ceiling_bps(tier: Tier) -> u64` helper using `QualityProfile::total_bitrate_kbps()`.
+  - In `try_transition()`, before upgrading to a higher tier, check BWE headroom:
+    ```rust
+    if let Some(ref bwe) = self.bwe {
+        let required = (Self::tier_ceiling_bps(next_tier) * 130) / 100;
+        if bwe.target_send_bps() < required {
+            self.consecutive_up = 0;
+            return None;
+        }
+    }
+    ```
+    This requires `target_send_bps() >= 130%` of the next tier's bitrate ceiling (including FEC overhead).
+
+## Why these choices
+
+The 130% headroom factor is a safety margin: we only upgrade if the bandwidth estimate comfortably exceeds the target tier's requirement, preventing flapping when BWE is borderline. Resetting `consecutive_up` to 0 on BWE block gives the estimator time to converge before the next upgrade attempt.
+
+Checking the *next* tier's ceiling (not the current tier) is correct: the guard answers "can we afford the tier we're trying to move into?"
+
+## Deviations from the task spec
+
+None.
+
+## Verification output
+
+```bash
+$ cargo test -p wzp-proto quality
+running 24 tests
+...(all 24 pass)...
+
+test result: ok. 24 passed; 0 failed; 0 ignored; 0 measured; 95 filtered out; finished in 0.10s
+```
+
+## Test summary
+
+- Tests added: 1
+  - `bwe_guard_blocks_upgrade_when_bandwidth_insufficient` — verifies low BWE blocks upgrade, high BWE allows it after counter reset
+- Tests modified: 0
+- `wzp-proto` test count: 119 (was 118 before T2.3)
+- `cargo clippy -p wzp-proto --all-targets -- -D warnings`: pass
+- `cargo fmt --all -- --check`: pass
+
+## Risks / follow-ups
+
+- `BandwidthEstimator` is attached via `set_bandwidth_estimator()`; call sites in `wzp-client` (send/recv loops) will create and wire it in a future task.
+- The BWE guard only applies to upgrades. Downgrades are unchanged — they react quickly to quality reports regardless of BWE.
+
+## Reviewer checklist (filled in by reviewer)
+
+- [ ] Code matches PRD intent
+- [ ] Verification output is real
+- [ ] No backward-incompat surprises
+- [ ] Tests cover the new behavior
+- [ ] Approved