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>

core/src/session/session.h

@@ -9,6 +9,7 @@
 
 #include <cstdint>
 #include <string>
+#include <utility>
 #include <vector>
 
 #ifdef VOICECAT_HAS_NET
@@ -30,6 +31,8 @@ struct Stream {
     uint32_t    ssrc{0};
     int         kind{0};
     std::string label;
+    uint32_t    sample_rate{48000};
+    uint32_t    frame_ms{20};
 };
 
 struct User {
@@ -48,6 +51,10 @@ class SessionModel {
     const Channel* find_channel(uint32_t id) const;
     const User*    find_user(uint32_t id) const;
 
+    // Find the user that owns a given media ssrc, and the matching Stream entry.
+    // Returns {nullptr, nullptr} if not found.
+    std::pair<const User*, const Stream*> find_user_by_ssrc(uint32_t ssrc) const;
+
 #ifdef VOICECAT_HAS_NET
     void apply_snapshot(const voicecat::v1::ServerStateSnapshot& snap);
     void apply_user_event(const voicecat::v1::UserEvent& ev);