G3.1: dedicated bridge heartbeat — watchdog liveness independent of audio loop

native_bridge/tests/shm_selfcheck.cpp

@@ -65,6 +65,7 @@ int main() {
     // 5. Layout stability — MUST match Rust src-tauri/src/shm.rs
     assert(sizeof(SharedAudioBufferIPC::MidiEventIPC) == 12);
     assert(sizeof(SharedAudioBufferIPC::ControlEventIPC) == 1040);
+    assert(sizeof(SharedAudioBufferIPC) == 11168); // G3.1: +4 heartbeat
 
     std::printf("SHM self-check OK (sizeof struct = %zu bytes)\n", sizeof(SharedAudioBufferIPC));
     shm_close(h);