feat: fix voice join/leave, channel edit defaults, channel-update stream restart

Three bugs fixed across the full stack (proto/server/core/ABI/Win/macOS/iOS):

1. Join/Leave Voice now truly subscribes/unsubscribes from the voice plane.
   Previously the button only toggled the local mic — receiving was always on
   (gated by channel membership alone). Added a protocol-level voice subscription
   concept: new SubscribeVoiceRequest/UnsubscribeVoiceRequest/VoiceSubscriptionResult
   proto messages, User.voice_subscribed field, vc_join_voice/vc_leave_voice C ABI
   functions, VC_EVENT_VOICE_STATE event, server-side voice_subscribed flag checked
   by the SFU relay recipient filter, and core-client gating of remote-stream
   decoder setup. All three clients rewired to subscribe+mic on Join / unsubscribe
   on Leave. Text chat works regardless of voice subscription.

2. Channel edit dialog now shows the channel's actual current settings. The read
   struct vc_channel was missing sort_order and audio fields — only the write
   struct vc_channel_info had them. Extended vc_channel with both (additive, no
   ABI break), updated the session model and list_channels marshaling to populate
   them, and updated all three clients' edit callers to use actual channel info
   instead of hardcoded defaults.

3. Channel parameter updates now automatically restart everyone's streams.
   Previously editing a channel's audio config persisted and broadcast a
   ChannelEvent::UPDATED, but no layer restarted streams — encoders/decoders are
   frozen at announce time. handle_channel_event now detects audio-config changes
   on the user's current channel and stop->starts each active local stream. The
   server reads the updated config on re-announce; peers wire up fresh decoders
   at the new ssrc.

All 29 CTest tests pass; Windows DLL + C# client build clean. Apple clients not
yet compile-verified (Windows environment).

server/src/conn_session.h

@@ -82,6 +82,7 @@ class ConnSession : public std::enable_shared_from_this<ConnSession> {
     State    state()      const { return state_.load(); }
     uint64_t session_id() const { return session_id_; }
     uint32_t user_id()    const { return user_id_; }
+    bool     voice_subscribed() const { return voice_subscribed_.load(std::memory_order_acquire); }
 
     // Keepalive: bump last_seen to "now" on any inbound activity (TCP frame or UDP voice
     // frame). The server's reaper sweeps sessions whose last_seen is older than the
@@ -105,6 +106,8 @@ class ConnSession : public std::enable_shared_from_this<ConnSession> {
     void handle_stream_announce(uint64_t req_id, const voicecat::v1::StreamAnnounce& msg);
     void handle_stream_stop(const voicecat::v1::StreamStop& msg);
     void handle_leave_channel();
+    void handle_subscribe_voice(uint64_t req_id);
+    void handle_unsubscribe_voice(uint64_t req_id);
 
     // M5 handlers
     void handle_kick_request(uint64_t req_id, const voicecat::v1::KickRequest& msg);
@@ -172,6 +175,11 @@ class ConnSession : public std::enable_shared_from_this<ConnSession> {
 
     // M5: permissions granted at auth time (server-side authority).
     voicecat::v1::Permissions permissions_;
+
+    // Voice-plane subscription. When false, the SFU relay excludes this session from the
+    // recipient set (find_channel_sessions), and handle_stream_announce rejects new streams.
+    // Toggled by SubscribeVoiceRequest / UnsubscribeVoiceRequest.
+    std::atomic<bool> voice_subscribed_{false};
 };
 
 }  // namespace voicecat::server