feat: per-session metrics + inter-relay health probe (T5-S2/S5)

WZP-P2-T5-S2: Per-session Prometheus metrics
- 5 new per-session gauges/counters: buffer_depth, loss_pct, rtt_ms,
  underruns, overruns — all labeled by session_id
- update_session_quality() reads QualityReport from packet headers
- update_session_buffer() tracks jitter buffer state per session
- remove_session_metrics() cleans up labels on disconnect
- Delta-aware counter increments avoid double-counting
- 2 tests: session_quality_update, session_metrics_cleanup

WZP-P2-T5-S5: Inter-relay health probe
- New probe.rs: ProbeConfig, ProbeMetrics, SlidingWindow, ProbeRunner
- --probe <addr> flag (repeatable) spawns background probe per target
- Sends Ping/s over QUIC, receives Pong, computes RTT/loss/jitter
- SlidingWindow(60): tracks last 60 pings, loss = missed pongs,
  jitter = std deviation of RTT
- Prometheus gauges: wzp_probe_rtt_ms, loss_pct, jitter_ms, up
  with target label
- Probe connections use SNI "_probe" — relay responds with Pong loop,
  skipping auth/handshake
- Auto-reconnect with 5s backoff on disconnect
- 6 tests: metrics_register, rtt/loss/jitter calculation,
  window eviction, empty edge cases

231 tests passing across all crates.

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

crates/wzp-relay/src/room.rs

@@ -179,6 +179,7 @@ pub async fn run_participant(
     participant_id: ParticipantId,
     transport: Arc<wzp_transport::QuinnTransport>,
     metrics: Arc<RelayMetrics>,
+    session_id: &str,
 ) {
     let addr = transport.connection().remote_address();
     let mut packets_forwarded = 0u64;
@@ -196,6 +197,11 @@ pub async fn run_participant(
             }
         };
 
+        // Update per-session quality metrics if a quality report is present
+        if let Some(ref report) = pkt.quality_report {
+            metrics.update_session_quality(session_id, report);
+        }
+
         // Get current list of other participants
         let others = {
             let mgr = room_mgr.lock().await;