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:
```
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

$ 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

2.5 KiB Raw Blame History

T2.3 — Plumb BWE into AdaptiveQualityController