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-relay/src/lib.rs

@@ -8,9 +8,11 @@
 //! quality transitions.
 
 pub mod config;
+pub mod handshake;
 pub mod pipeline;
 pub mod session_mgr;
 
 pub use config::RelayConfig;
+pub use handshake::accept_handshake;
 pub use pipeline::{PipelineConfig, PipelineStats, RelayPipeline};
 pub use session_mgr::{RelaySession, SessionId, SessionManager};