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

native_bridge/include/SharedMemoryIPC.h

@@ -55,4 +55,10 @@ struct SharedAudioBufferIPC {
     // to extend its stall deadline — a long Nexus attach must not look like
     // a dead bridge. Append-only field: existing offsets stay stable.
     volatile uint32_t attachInFlight;
+
+    // G3.1: dedicated heartbeat, incremented every 100ms by a bridge-side
+    // thread INDEPENDENT of the audio loop. The Tauri watchdog keys bridge
+    // liveness on this, so a slow (but alive) audio loop no longer looks dead.
+    // Append-only field: existing offsets stay stable.
+    volatile uint32_t heartbeat;
 };

native_bridge/src/main.cpp

@@ -657,6 +657,17 @@ int main(int argc, char* argv[]) {
     if (!shmIPC) return 1;
 #endif
 
+    // G3.1: dedicated heartbeat thread, independent of the audio loop.
+    // Increments every 100ms (~10Hz). Tauri watchdog keys liveness on this.
+    // Started right after the SHM map so a slow instrument restore at
+    // startup (or after watchdog respawn) never looks like a dead bridge.
+    std::thread([shmIPC]() {
+        for (;;) {
+            Sleep(100);
+            shmIPC->heartbeat++;
+        }
+    }).detach();
+
     InstrumentEngineManager instruments;
 #ifdef _WIN32
     g_engine = &instruments;

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);