fix(media): stop permanent voice loss after bad-network blip (protocol v2)

A bad UDP packet on a flaky link could permanently wedge the voice path,
unrecoverable even across app restarts. Three defects:

1. Anti-replay window was advanced from the UNAUTHENTICATED header seq
   before the AEAD tag was checked, and not rolled back on failure. One
   corrupted/forged frame shoved recv_highest_ far ahead, after which every
   legitimate frame was rejected as "too old" forever. Reorder to
   replay-check -> authenticate -> update (RFC 3711 3.3); the window now
   moves only after a successful tag check.

2. The wire seq was only the low 16 bits of the nonce counter (zero-extended
   on receive). After 65,536 frames the nonce desynced and all frames failed
   auth. Widen the voice frame seq u16 -> u64 (header 14 -> 20 bytes). The
   core owns all UDP framing, so Swift/C# clients need only a rebuild. This
   is a versioned wire change: VOICECAT_PROTOCOL_VERSION 1 -> 2, handshake
   rejects on mismatch.

3. Server leaked per-session UDP state on disconnect; unregister_session now
   frees udp_endpoints_/udp_tokens_/ssrc_to_session_.

Also add rate-limited dropped-frame logging to MediaRelay so a wedged media
path is observable. New regression tests in test_media_aead.cpp cover the
poison (fails on old code) and the 16-bit wrap. ctest --preset dev
-E external_pcm: 22/22 pass (external_pcm aborts on a pre-existing CoreAudio
shutdown race, unrelated).

server/src/conn_session.cpp

@@ -257,13 +257,13 @@ void ConnSession::send_generic_result(uint64_t req_id, bool ok, uint32_t code,
 // ── Handlers ─────────────────────────────────────────────────────────────────
 
 void ConnSession::handle_client_hello(uint64_t req_id, const voicecat::v1::ClientHello& msg) {
-    if (msg.proto_version() != 1) {
+    if (msg.proto_version() != 2) {  // protocol v2: 64-bit voice seq (docs/voice.md §2)
         send_disconnect_and_close(1, "unsupported protocol version");
         return;
     }
     auto env = make_env(req_id);
     auto* hello = env.mutable_server_hello();
-    hello->set_proto_version(1);
+    hello->set_proto_version(2);
     hello->set_server_name("VoiceCat Server");
     hello->set_server_version("0.1.0");
     if (allow_guests_) hello->add_auth_methods("guest");