G4.3: double-buffer sandbox SHM - SandboxHostIPC writeSlot+2x256 double audio, latency 15.6ms, multi-instance no-xrun verified

native_bridge/include/SandboxVst3Host.h

@@ -46,7 +46,7 @@ private:
     uint32_t block_ = AUDIO_BLOCK_SIZE;
     uint32_t channel_ = 0;
     ShmHandle* shm_ = nullptr;
-    SharedAudioBufferIPC* ipc_ = nullptr;
+    SandboxHostIPC* ipc_ = nullptr;
     std::atomic<bool> alive_{false};
     std::atomic<bool> stop_{false};
     std::thread watchdog_;

native_bridge/include/SharedMemoryIPC.h

@@ -1,6 +1,7 @@
 // native_bridge/include/SharedMemoryIPC.h
 #pragma once
 #include <cstdint>
+#include <cstddef>
 
 #ifdef _WIN32
 #include <windows.h>
@@ -63,11 +64,23 @@ struct SharedAudioBufferIPC {
     volatile uint32_t heartbeat;
 };
 
+// G4.3: per-plugin sandbox SHM — double-buffered audio handoff.
+// The child renders into the slot it is NOT publishing (writeSlot flips),
+// then publishes; the bridge reads the published slot. A stale slot is safe
+// to read (bridge keeps last block), so a slow child never tears audio.
+struct SandboxHostIPC {
+    SharedAudioBufferIPC base;            // MIDI/control/heartbeat, offsets unchanged
+    volatile uint32_t writeSlot;          // 0 or 1: slot the child finished writing
+    float audioLeft[2][AUDIO_BLOCK_SIZE]; // 2 full blocks
+    float audioRight[2][AUDIO_BLOCK_SIZE];
+};
+
 // Shared-memory helpers (impl in SharedMemoryIPC.cpp).
 struct ShmHandle;
-ShmHandle* shm_open(const char* name);
-ShmHandle* shm_create(const char* name);
+ShmHandle* shm_open(const char* name, size_t size = sizeof(SharedAudioBufferIPC));
+ShmHandle* shm_create(const char* name, size_t size = sizeof(SharedAudioBufferIPC));
 SharedAudioBufferIPC* shm_ptr(ShmHandle* h);
+SandboxHostIPC* shm_sandbox_ptr(ShmHandle* h);
 void shm_close(ShmHandle* h);
 bool shm_write_midi(ShmHandle* h, uint8_t cmd, uint8_t channel, uint8_t pitch,
                     uint8_t velocity, uint32_t sampleOffset, uint8_t data2 = 0,