feat: Phase 3 — crypto handshake, codec2, benchmarks, audio I/O, relay forwarding

E2E crypto handshake:
- Client/relay handshake via SignalMessage (CallOffer/CallAnswer)
- X25519 ephemeral key exchange with Ed25519 identity signatures
- Integration tests proving bidirectional encrypt/decrypt

Codec2 integration:
- Pure Rust codec2 crate (v0.3) — no C bindings needed
- MODE_3200 (160 samples/20ms, 8 bytes) and MODE_1200 (320 samples/40ms, 6 bytes)
- 11 new tests including encode/decode roundtrip and adaptive switching

Relay forwarding:
- Bidirectional client → remote forwarding with pipeline processing
- CLI args: --listen, --remote
- Periodic stats logging, clean shutdown via tokio::select!

Benchmark tool (wzp-bench):
- Codec roundtrip, FEC recovery, crypto throughput, full pipeline benchmarks
- Sine wave PCM generator for realistic testing

Audio I/O (cpal):
- AudioCapture (microphone) and AudioPlayback (speakers) at 48kHz mono
- CLI --live mode: mic → encode → send / recv → decode → speakers

120 tests passing, 0 failures.

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

crates/wzp-client/Cargo.toml

@@ -18,9 +18,19 @@ tracing-subscriber = { workspace = true }
 async-trait = { workspace = true }
 bytes = { workspace = true }
 anyhow = "1"
+cpal = "0.15"
 
 [[bin]]
 name = "wzp-client"
 path = "src/cli.rs"
 
+[[bin]]
+name = "wzp-bench"
+path = "src/bench_cli.rs"
+
 [dev-dependencies]
+tokio = { workspace = true }
+wzp-relay = { path = "../wzp-relay" }
+wzp-crypto = { workspace = true }
+wzp-proto = { workspace = true }
+async-trait = { workspace = true }