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>

Cargo.lock

@@ -138,13 +138,43 @@ dependencies = [
  "fs_extra",
 ]
 
+[[package]]
+name = "axum"
+version = "0.7.9"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "edca88bc138befd0323b20752846e6587272d3b03b0343c8ea28a6f819e6e71f"
+dependencies = [
+ "async-trait",
+ "axum-core 0.4.5",
+ "bytes",
+ "futures-util",
+ "http",
+ "http-body",
+ "http-body-util",
+ "hyper",
+ "hyper-util",
+ "itoa",
+ "matchit 0.7.3",
+ "memchr",
+ "mime",
+ "percent-encoding",
+ "pin-project-lite",
+ "rustversion",
+ "serde",
+ "sync_wrapper",
+ "tokio",
+ "tower",
+ "tower-layer",
+ "tower-service",
+]
+
 [[package]]
 name = "axum"
 version = "0.8.8"
 source = "registry+https://github.com/rust-lang/crates.io-index"
 checksum = "8b52af3cb4058c895d37317bb27508dccc8e5f2d39454016b297bf4a400597b8"
 dependencies = [
- "axum-core",
+ "axum-core 0.5.6",
  "base64",
  "bytes",
  "form_urlencoded",
@@ -155,7 +185,7 @@ dependencies = [
  "hyper",
  "hyper-util",
  "itoa",
- "matchit",
+ "matchit 0.8.4",
  "memchr",
  "mime",
  "percent-encoding",
@@ -174,6 +204,26 @@ dependencies = [
  "tracing",
 ]
 
+[[package]]
+name = "axum-core"
+version = "0.4.5"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "09f2bd6146b97ae3359fa0cc6d6b376d9539582c7b4220f041a33ec24c226199"
+dependencies = [
+ "async-trait",
+ "bytes",
+ "futures-util",
+ "http",
+ "http-body",
+ "http-body-util",
+ "mime",
+ "pin-project-lite",
+ "rustversion",
+ "sync_wrapper",
+ "tower-layer",
+ "tower-service",
+]
+
 [[package]]
 name = "axum-core"
 version = "0.5.6"
@@ -1521,6 +1571,12 @@ dependencies = [
  "libc",
 ]
 
+[[package]]
+name = "matchit"
+version = "0.7.3"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "0e7465ac9959cc2b1404e8e2367b43684a6d13790fe23056cc8c6c5a6b7bcb94"
+
 [[package]]
 name = "matchit"
 version = "0.8.4"
@@ -1888,6 +1944,27 @@ dependencies = [
  "unicode-ident",
 ]
 
+[[package]]
+name = "prometheus"
+version = "0.13.4"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "3d33c28a30771f7f96db69893f78b857f7450d7e0237e9c8fc6427a81bae7ed1"
+dependencies = [
+ "cfg-if",
+ "fnv",
+ "lazy_static",
+ "memchr",
+ "parking_lot",
+ "protobuf",
+ "thiserror 1.0.69",
+]
+
+[[package]]
+name = "protobuf"
+version = "2.28.0"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "106dd99e98437432fed6519dedecfade6a06a73bb7b2a1e019fdd2bee5778d94"
+
 [[package]]
 name = "quinn"
 version = "0.11.9"
@@ -2591,7 +2668,7 @@ source = "registry+https://github.com/rust-lang/crates.io-index"
 checksum = "32497e9a4c7b38532efcdebeef879707aa9f794296a4f0244f6f69e9bc8574bd"
 dependencies = [
  "fastrand",
- "getrandom 0.3.4",
+ "getrandom 0.4.2",
  "once_cell",
  "rustix",
  "windows-sys 0.61.2",
@@ -3722,6 +3799,7 @@ dependencies = [
  "anyhow",
  "async-trait",
  "bytes",
+ "chrono",
  "cpal",
  "serde",
  "serde_json",
@@ -3796,7 +3874,9 @@ version = "0.1.0"
 dependencies = [
  "anyhow",
  "async-trait",
+ "axum 0.7.9",
  "bytes",
+ "prometheus",
  "quinn",
  "reqwest",
  "rustls",
@@ -3834,10 +3914,11 @@ name = "wzp-web"
 version = "0.1.0"
 dependencies = [
  "anyhow",
- "axum",
+ "axum 0.8.8",
  "axum-server",
  "bytes",
  "futures",
+ "prometheus",
  "rcgen",
  "rustls",
  "rustls-pemfile",