feat(M2): UDP voice/media plane -- SFU relay, Opus, AEAD, jitter buffer

Adds the full voice pipeline: 14-byte binary frame header, ChaCha20-Poly1305
AEAD keyed from the TLS exporter, libopus encode/decode with FEC/PLC/DTX,
an adaptive per-ssrc jitter buffer, a miniaudio capture/playback engine, an
APM passthrough stub, and the UdpBinding/StreamAnnounce signaling chain
wired through ConnSession/SessionRegistry into a new server-side SFU
(MediaRelay) that decrypts and re-encrypts frames per channel member.

Exit criterion verified: test_m2_voice — two headless clients relay 50
encrypted Opus frames through the server; ctest --preset m1-dev is 9/9
green. Also corrects protocol.md's UdpBinding diagram, which described the
UDP-side binding packet as AEAD-sealed when it is in fact a plaintext
bootstrap frame (separate from the TCP/TLS UdpBinding ack).

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

core/src/net/transport.h

@@ -40,6 +40,9 @@ struct TcpChannelCallbacks {
     std::function<void(std::vector<uint8_t>)>      on_frame;   // one decoded frame payload
     std::function<void(std::error_code)>           on_error;
     std::function<void()>                          on_disconnected;
+    // Called (on the handshake thread) right after TLS succeeds, before reads begin.
+    // Use to export keying material while the handshake context is still fresh.
+    std::function<void(voicecat::crypto::TlsContext&)> on_tls_ready;
 };
 
 // ── Client-side: owns an io_context + dedicated net thread ──────────────────
@@ -166,12 +169,44 @@ class TcpAcceptor {
 };
 
 // ── UDP media channel (M2) ───────────────────────────────────────────────────
+// Thin async UDP socket. send_to() is thread-safe. Recv callbacks fire on the
+// io_context's thread (same thread that runs the io_context::run() loop).
 class UdpMediaChannel {
    public:
-    bool bound() const { return bound_; }
+    using FrameCallback =
+        std::function<void(const uint8_t*, size_t, asio::ip::udp::endpoint)>;
+
+    UdpMediaChannel() = default;
+    ~UdpMediaChannel() { close(); }
+
+    UdpMediaChannel(const UdpMediaChannel&)            = delete;
+    UdpMediaChannel& operator=(const UdpMediaChannel&) = delete;
+
+    // Bind to 0.0.0.0:port (0 = OS-assigned). Must be called before start_recv/send_to.
+    bool bind(asio::io_context& io, uint16_t port = 0);
+
+    // Begin the async recv loop. cb is called on the io_context thread.
+    void start_recv(FrameCallback cb);
+
+    // Thread-safe fire-and-forget send. Copies data into a heap buffer.
+    void send_to(const uint8_t* data, size_t len, asio::ip::udp::endpoint dst);
+
+    // Cancel all async ops and close the socket. Safe to call from any thread.
+    void close();
+
+    asio::ip::udp::endpoint local_endpoint() const;
+    bool bound() const { return bound_.load(std::memory_order_acquire); }
 
    private:
-    bool bound_ = false;
+    void do_recv();
+
+    // Socket + recv state live here; only accessed from the io_context thread after bind().
+    std::unique_ptr<asio::ip::udp::socket>  socket_;
+    asio::ip::udp::endpoint                 sender_ep_;
+    std::array<uint8_t, 1500>               recv_buf_{};
+    FrameCallback                           frame_cb_;
+    std::atomic<bool>                       bound_{false};
+    std::atomic<bool>                       closed_{false};
 };
 
 }  // namespace voicecat::net