fix(M2): wire vc_client's real voice plane through the C ABI, not just raw sockets

test_m2_voice passed against raw BSD sockets, but vc_client::stream_start/stop,
UDP binding, and capture/recv were still VC_ERR_NOT_IMPLEMENTED stubs -- meaning
vccli and any GUI client still couldn't actually talk. Implements the real
client-side UDP-binding handshake, media key derivation, capture->encode->seal->
send and recv->open->decode->playback paths, plus server-side StreamInfo
broadcast so peers learn about each other's streams via sync_remote_streams().

Adds test_voice_client_abi (two real vc_client instances, not raw sockets) and
vccli --voice/--mute/--text flags, manually verified live between two instances.

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

server/src/session_registry.h

@@ -14,6 +14,7 @@
 #include <atomic>
 #include <cstdint>
 #include <memory>
+#include <optional>
 #include <shared_mutex>
 #include <string>
 #include <unordered_map>
@@ -95,6 +96,15 @@ class SessionRegistry {
     // Assign an SSRC for a new stream. Returns the assigned SSRC.
     uint32_t assign_ssrc(uint64_t session_id);
 
+    // Add/replace a stream entry on a user (called when StreamAnnounce succeeds).
+    // Returns the updated User proto for broadcasting, or nullopt if user not found.
+    std::optional<voicecat::v1::User> set_user_stream(uint32_t user_id,
+                                                        const voicecat::v1::StreamInfo& info);
+
+    // Remove a stream entry from a user (called on StreamStop). Returns the updated
+    // User proto for broadcasting, or nullopt if user not found.
+    std::optional<voicecat::v1::User> clear_user_stream(uint32_t user_id, uint32_t stream_id);
+
     // Get all sessions in a channel except the one excluded (for SFU relay).
     std::vector<std::shared_ptr<ConnSession>> find_channel_sessions(
         uint32_t channel_id, uint64_t exclude_session_id = 0) const;