feat(M1): TCP/TLS control plane -- auth, channels, ephemeral text

Implements the full M1 milestone. Two clients authenticate over TLS 1.3
(guest + Argon2id password) and exchange channel + private text messages
through a real server. All five ctest --preset m1-dev tests pass in ~1 s.

Key components added:
- vcpkg baseline + m1-dev preset (protobuf/mbedTLS/libsodium/asio/sqlite3)
- FrameCodec feed+emit, encode/decode_envelope, protobuf codegen
- TcpServerConn with blocking TLS handshake thread + tls_read_loop
- TlsContext (mbedTLS 1.3, ECDSA-P256 self-signed cert, TOFU on client)
- WorkerPool (3 threads, used for Argon2id)
- Database: SQLite + libsodium Argon2id, account lifecycle, bootstrap admin
- ServerIdentityManager: Ed25519 key + cert generate/persist/fingerprint
- ConnSession state machine: WaitingHello -> WaitingAuth -> Authenticated
- SessionRegistry: channel tree, user map, text routing, broadcast
- vc_client full M1 C ABI: connect/TLS/handshake/auth/text/disconnect
- voicecat-admin CLI: account add/reset/del/list
- test_m1_integration: M1 exit criterion, verified green

Bug fixed: double-framing in ConnSession::send_envelope -- encode_envelope
was adding the [4-byte len] prefix, then TcpServerConn::send_frame added
a second one, causing the client to parse [len][proto] as protobuf (silent
failure). Fixed by serializing raw protobuf bytes in send_envelope and
letting send_frame apply the single length prefix.

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

core/src/core/client.h

@@ -1,16 +1,30 @@
 /*
  * client.h — the implementation type behind the opaque `vc_client*` handle.
- *
- * M0 skeleton: holds config/callbacks/state and returns VC_ERR_NOT_IMPLEMENTED for
- * everything that needs a subsystem. As subsystems land (docs/architecture.md §2), this
- * class wires them together: a net transport, a protocol state machine, a session model,
- * and an audio engine, plus the event thread that drains into vc_callbacks.on_event.
  */
 #ifndef VOICECAT_CORE_CLIENT_H
 #define VOICECAT_CORE_CLIENT_H
 
 #include "voicecat.h"
 
+#ifdef VOICECAT_HAS_NET
+
+#include <atomic>
+#include <condition_variable>
+#include <deque>
+#include <mutex>
+#include <optional>
+#include <string>
+#include <thread>
+#include <vector>
+
+#include "crypto/crypto.h"
+#include "protocol/envelope.h"
+#include "protocol/protocol.h"
+#include "session/session.h"
+#include "proto/voicecat.pb.h"
+
+#endif  // VOICECAT_HAS_NET
+
 struct vc_client {
     vc_client(const vc_config& cfg, vc_callbacks cb);
     ~vc_client();
@@ -39,15 +53,85 @@ struct vc_client {
 
     vc_result list_devices(vc_device_kind kind, vc_device_list* out);
 
-    vc_connection_state state() const { return state_; }
+    vc_connection_state state() const {
+#ifdef VOICECAT_HAS_NET
+        return state_net_.load(std::memory_order_acquire);
+#else
+        return state_;
+#endif
+    }
 
    private:
-    // Deliver an event to the host application. Safe to call with cb_.on_event == nullptr.
     void emit(const vc_event& ev) const;
 
-    vc_config cfg_{};
+    vc_config    cfg_{};
     vc_callbacks cb_{};
-    vc_connection_state state_ = VC_STATE_DISCONNECTED;
+
+#ifdef VOICECAT_HAS_NET
+    // ── M1: TCP/TLS control channel ─────────────────────────────────────────────
+    std::atomic<vc_connection_state> state_net_{VC_STATE_DISCONNECTED};
+
+    // Blocking I/O thread (one per vc_client lifetime)
+    std::thread      io_thread_;
+    std::atomic<bool> io_stop_{false};
+
+    // Send queue: pushed by any thread, drained by io_thread_
+    std::mutex                        send_mutex_;
+    std::condition_variable           send_cv_;
+    std::deque<std::vector<uint8_t>>  send_queue_;
+
+    // TLS context — created + used exclusively on io_thread_
+    std::unique_ptr<voicecat::crypto::TlsContext> tls_;
+
+    // Raw socket fd (stored after TCP connect; closed by disconnect())
+    std::atomic<int> io_fd_{-1};
+
+    // Pending auth stored before ServerHello arrives
+    struct PendingAuth {
+        bool        is_guest{true};
+        std::string nick_or_user;
+        std::string password;
+    };
+    std::mutex                    pending_auth_mutex_;
+    std::optional<PendingAuth>    pending_auth_;
+
+    // Self identity filled in after AuthResult
+    uint32_t self_user_id_{0};
+    uint64_t server_session_id_{0};
+    std::atomic<uint64_t> next_req_id_{1};
+
+    // Client-side session model
+    voicecat::session::SessionModel session_model_;
+
+    // ── io_thread_ entry point ──────────────────────────────────────────────────
+    void run_io(std::string host, uint16_t port);
+
+    // ── Protocol dispatch (called on io_thread_) ────────────────────────────────
+    void handle_envelope(const voicecat::v1::Envelope& env);
+    void handle_server_hello(const voicecat::v1::ServerHello& msg, uint64_t req_id);
+    void handle_auth_result(const voicecat::v1::AuthResult& msg);
+    void handle_server_state(const voicecat::v1::ServerStateSnapshot& snap);
+    void handle_user_event(const voicecat::v1::UserEvent& ue);
+    void handle_text_message(const voicecat::v1::TextMessage& msg);
+    void handle_disconnect(const voicecat::v1::Disconnect& msg);
+
+    // ── Helpers (io_thread_ and caller threads) ─────────────────────────────────
+    // Queue an encoded envelope to be sent on io_thread_.
+    void queue_envelope(const voicecat::v1::Envelope& env);
+
+    // Drain send_queue_ by doing blocking TLS writes (called on io_thread_).
+    void drain_sends();
+
+    // Transition state + emit VC_EVENT_CONNECTION_STATE.
+    void set_state(vc_connection_state s);
+
+    // Convenience event emitters.
+    void emit_error(vc_result r, const char* text);
+    void emit_disconnected(vc_result r, const char* reason);
+
+#else  // !VOICECAT_HAS_NET
+    vc_connection_state state_{VC_STATE_DISCONNECTED};
+#endif
 };
 
 #endif  // VOICECAT_CORE_CLIENT_H