feat: Prometheus metrics on relay + web bridge, client JSONL export (T5-S1/S3/S4)

WZP-P2-T5-S1: Relay Prometheus /metrics
- RelayMetrics: active_sessions, active_rooms, packets/bytes_forwarded,
  auth_attempts (ok/fail), handshake_duration histogram
- --metrics-port flag spawns HTTP server
- Wired into auth, handshake, session, and packet forwarding paths
- 2 tests

WZP-P2-T5-S3: Web bridge Prometheus /metrics
- WebMetrics: active_connections, frames_bridged (up/down),
  auth_failures, handshake_latency histogram
- Added /metrics route to existing axum app
- Wired into WS connect/disconnect, auth, handshake, send/recv loops
- 2 tests

WZP-P2-T5-S4: Client --metrics-file JSONL
- ClientMetricsSnapshot with all telemetry fields
- MetricsWriter: writes one JSON line per second to file
- snapshot_from_stats() converts JitterStats to snapshot
- --metrics-file <path> flag
- 3 tests

223 tests passing across all crates.

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

crates/wzp-relay/src/config.rs

@@ -22,6 +22,9 @@ pub struct RelayConfig {
     /// featherChat auth validation URL (e.g., "https://chat.example.com/v1/auth/validate").
     /// If set, clients must present a valid token before joining rooms.
     pub auth_url: Option<String>,
+    /// Port for the Prometheus metrics HTTP endpoint (e.g., 9090).
+    /// If None, the metrics endpoint is disabled.
+    pub metrics_port: Option<u16>,
 }
 
 impl Default for RelayConfig {
@@ -34,6 +37,7 @@ impl Default for RelayConfig {
             jitter_max_depth: 250,
             log_level: "info".to_string(),
             auth_url: None,
+            metrics_port: None,
         }
     }
 }