feat: 5-tier quality classification, QualityDirective handling, debug tap stats

- Extend Tier enum from 3 to 6 levels: Studio64k/48k/32k + Good +
  Degraded + Catastrophic with asymmetric hysteresis (down:3, up:5,
  studio:10)
- Handle QualityDirective signals in both desktop and Android engines
  — relay-coordinated codec switching now works end-to-end
- Add periodic TAP STATS to debug tap: packets in/out, fan-out avg,
  seq gaps, codecs seen (every 5s)
- Mark task #2 done (ParticipantInfo in federation signals already
  implemented)

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

desktop/src-tauri/src/engine.rs

@@ -1114,10 +1114,11 @@ impl CallEngine {
             }
         });
 
-        // Signal task (presence — same shape as desktop).
+        // Signal task (presence + quality directives).
         let sig_t = transport.clone();
         let sig_r = running.clone();
         let sig_p = participants.clone();
+        let sig_pending_profile = pending_profile.clone();
         let event_cb = Arc::new(event_cb);
         let sig_cb = event_cb.clone();
         tokio::spawn(async move {
@@ -1149,6 +1150,18 @@ impl CallEngine {
                         *sig_p.lock().await = unique;
                         sig_cb("room-update", &format!("{count} participants"));
                     }
+                    Ok(Ok(Some(wzp_proto::SignalMessage::QualityDirective {
+                        recommended_profile,
+                        reason,
+                    }))) => {
+                        let idx = profile_to_index(&recommended_profile);
+                        info!(
+                            codec = ?recommended_profile.codec,
+                            reason = reason.as_deref().unwrap_or(""),
+                            "relay quality directive: switching profile"
+                        );
+                        sig_pending_profile.store(idx, Ordering::Release);
+                    }
                     Ok(Ok(Some(_))) => {}
                     Ok(Ok(None)) => break,
                     Ok(Err(_)) => break,
@@ -1534,10 +1547,11 @@ impl CallEngine {
             }
         });
 
-        // Signal task (presence)
+        // Signal task (presence + quality directives)
         let sig_t = transport.clone();
         let sig_r = running.clone();
         let sig_p = participants.clone();
+        let sig_pending_profile = pending_profile.clone();
         let event_cb = Arc::new(event_cb);
         let sig_cb = event_cb.clone();
         tokio::spawn(async move {
@@ -1569,6 +1583,18 @@ impl CallEngine {
                         *sig_p.lock().await = unique;
                         sig_cb("room-update", &format!("{count} participants"));
                     }
+                    Ok(Ok(Some(wzp_proto::SignalMessage::QualityDirective {
+                        recommended_profile,
+                        reason,
+                    }))) => {
+                        let idx = profile_to_index(&recommended_profile);
+                        info!(
+                            codec = ?recommended_profile.codec,
+                            reason = reason.as_deref().unwrap_or(""),
+                            "relay quality directive: switching profile"
+                        );
+                        sig_pending_profile.store(idx, Ordering::Release);
+                    }
                     Ok(Ok(Some(_))) => {}
                     Ok(Ok(None)) => break,
                     Ok(Err(_)) => break,