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/session/session.h

@@ -1,11 +1,8 @@
 /*
- * session/session.h — domain model: channels, users, streams, permissions, text.
+ * session/session.h — client-side mirror of the server's channel/user state.
  *
- * Design: docs/protocol.md §5, docs/architecture.md §5. Shared by client (local mirror of
- * server state) and server (authoritative). Text is ephemeral (no history). Accounts are
- * admin-provisioned.
- *
- * STATUS: M0 stub.
+ * Populated from ServerStateSnapshot (full snapshot) and incremental UserEvent /
+ * ChannelEvent messages. Not thread-safe — always called from io_thread_.
  */
 #ifndef VOICECAT_SESSION_SESSION_H
 #define VOICECAT_SESSION_SESSION_H
@@ -14,40 +11,52 @@
 #include <string>
 #include <vector>
 
+#ifdef VOICECAT_HAS_NET
+#include "proto/voicecat.pb.h"
+#endif
+
 namespace voicecat::session {
 
 struct Channel {
-    uint32_t id = 0;
-    uint32_t parent_id = 0;
+    uint32_t    id{0};
+    uint32_t    parent_id{0};
     std::string name;
-    bool password_protected = false;
-    uint32_t max_users = 0;
+    bool        password_protected{false};
+    uint32_t    max_users{0};
 };
 
 struct Stream {
-    uint32_t stream_id = 0;
-    uint32_t ssrc = 0;
-    int kind = 0;  // vc_stream_kind
+    uint32_t    stream_id{0};
+    uint32_t    ssrc{0};
+    int         kind{0};
     std::string label;
 };
 
 struct User {
-    uint32_t id = 0;
-    std::string nickname;
-    bool is_guest = true;
-    uint32_t channel_id = 0;
+    uint32_t            id{0};
+    std::string         nickname;
+    bool                is_guest{true};
+    uint32_t            channel_id{0};
     std::vector<Stream> streams;
 };
 
-// Mirror/authority for the channel tree + user list. TODO(M1): snapshot + delta apply.
 class SessionModel {
    public:
     const std::vector<Channel>& channels() const { return channels_; }
-    const std::vector<User>& users() const { return users_; }
+    const std::vector<User>&    users()    const { return users_; }
+
+    const Channel* find_channel(uint32_t id) const;
+    const User*    find_user(uint32_t id) const;
+
+#ifdef VOICECAT_HAS_NET
+    void apply_snapshot(const voicecat::v1::ServerStateSnapshot& snap);
+    void apply_user_event(const voicecat::v1::UserEvent& ev);
+    void apply_channel_event(const voicecat::v1::ChannelEvent& ev);
+#endif
 
    private:
     std::vector<Channel> channels_;
-    std::vector<User> users_;
+    std::vector<User>    users_;
 };
 
 }  // namespace voicecat::session