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/Cargo.toml

@@ -24,6 +24,8 @@ reqwest = { version = "0.12", features = ["json"] }
 serde_json = "1"
 rustls = { version = "0.23", default-features = false, features = ["ring", "std"] }
 quinn = { workspace = true }
+prometheus = "0.13"
+axum = { version = "0.7", default-features = false, features = ["tokio", "http1"] }
 
 [[bin]]
 name = "wzp-relay"