test+docs: add tests for QualityDirective, ParticipantQuality; update docs

- QualityDirective signal roundtrip tests (with/without reason)
- ParticipantQuality unit tests (initial tier, degradation, weakest-link)
- Updated PROGRESS.md with desktop adaptive quality, relay coordinated
  switching, Oboe state polling entries
- Updated ARCHITECTURE.md SFU fan-out rules with QualityDirective
- Updated PRD-coordinated-codec.md with implementation status
- 312 tests passing across all modified crates

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

crates/wzp-proto/src/packet.rs

@@ -1673,6 +1673,41 @@ mod tests {
         }
     }
 
+    #[test]
+    fn quality_directive_roundtrip() {
+        let msg = SignalMessage::QualityDirective {
+            recommended_profile: crate::QualityProfile::DEGRADED,
+            reason: Some("weakest link degraded".into()),
+        };
+        let json = serde_json::to_string(&msg).unwrap();
+        let decoded: SignalMessage = serde_json::from_str(&json).unwrap();
+        match decoded {
+            SignalMessage::QualityDirective { recommended_profile, reason } => {
+                assert_eq!(recommended_profile.codec, CodecId::Opus6k);
+                assert_eq!(reason.as_deref(), Some("weakest link degraded"));
+            }
+            _ => panic!("wrong variant"),
+        }
+    }
+
+    #[test]
+    fn quality_directive_without_reason_roundtrip() {
+        let msg = SignalMessage::QualityDirective {
+            recommended_profile: crate::QualityProfile::GOOD,
+            reason: None,
+        };
+        let json = serde_json::to_string(&msg).unwrap();
+        // None reason should be omitted from JSON
+        assert!(!json.contains("reason"));
+        let decoded: SignalMessage = serde_json::from_str(&json).unwrap();
+        match decoded {
+            SignalMessage::QualityDirective { reason, .. } => {
+                assert!(reason.is_none());
+            }
+            _ => panic!("wrong variant"),
+        }
+    }
+
     #[test]
     fn mini_frame_disabled() {
         // Simulate disabled mini-frames by always keeping frames_since_full at 0

crates/wzp-relay/src/room.rs

@@ -1099,4 +1099,47 @@ mod tests {
         // Batcher should now be empty — nothing to flush.
         assert!(batcher.flush().is_none());
     }
+
+    fn make_report(loss_pct_f: f32, rtt_ms: u16) -> wzp_proto::packet::QualityReport {
+        wzp_proto::packet::QualityReport {
+            loss_pct: (loss_pct_f / 100.0 * 255.0) as u8,
+            rtt_4ms: (rtt_ms / 4) as u8,
+            jitter_ms: 10,
+            bitrate_cap_kbps: 200,
+        }
+    }
+
+    #[test]
+    fn participant_quality_starts_good() {
+        let pq = ParticipantQuality::new();
+        assert_eq!(pq.current_tier, Tier::Good);
+    }
+
+    #[test]
+    fn participant_quality_degrades_on_bad_reports() {
+        let mut pq = ParticipantQuality::new();
+        let bad = make_report(50.0, 300);
+        // Feed enough bad reports to trigger downgrade (3 consecutive)
+        for _ in 0..5 {
+            pq.observe(&bad);
+        }
+        assert_ne!(pq.current_tier, Tier::Good, "should degrade from Good");
+    }
+
+    #[test]
+    fn weakest_tier_picks_worst() {
+        let good = ParticipantQuality::new();
+        // good stays at Good tier
+
+        let mut bad = ParticipantQuality::new();
+        let bad_report = make_report(50.0, 300);
+        for _ in 0..5 {
+            bad.observe(&bad_report);
+        }
+        // bad should be degraded or catastrophic
+
+        let participants = vec![good, bad];
+        let weakest = weakest_tier(participants.iter());
+        assert_ne!(weakest, Tier::Good, "weakest should not be Good when one participant is bad");
+    }
 }