fix(vsti): crackle fix P1-P3 — SHM 8-slot ring + drain-delta pump + AudioWorklet sink

- SharedMemoryIPC.h/main.cpp: render into ring slot (idx & ringMask), 8 slots
- shm.rs: ring fields + read_ring_audio; lib.rs pump drains FULL delta, not just latest
- bridgeAudioNode.js: AudioWorklet primary (bridge-audio-sink), ScriptProcessor fallback
- bridgeAudioWorklet.js: ring 64 blocks, underrun fade, overrun drop + crossfade
- layout tests: sizeof 27560 (8-align via u64)

native_bridge/include/SharedMemoryIPC.h

@@ -62,6 +62,17 @@ struct SharedAudioBufferIPC {
     // 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;
+
+    // VSTi crackle fix: multi-slot audio ring (appended after heartbeat — all
+    // offsets above stay stable). The bridge renders block N into slot
+    // (bridgeWriteIndex & ringMask), then increments bridgeWriteIndex (publish).
+    // The reader drains EVERY slot between its last index and writeIndex, in
+    // order, so a delayed reader no longer loses a whole block to single-slot
+    // overwrite (the old 1-slot gap). ringMask = slot count - 1 (power of 2);
+    // 8 slots ~= 42ms of audio @ 48k — covers a pump/WebView jank window.
+    volatile uint32_t ringMask;
+    float ringLeft[8][AUDIO_BLOCK_SIZE];
+    float ringRight[8][AUDIO_BLOCK_SIZE];
 };
 
 // G4.3: per-plugin sandbox SHM — double-buffered audio handoff.

native_bridge/src/main.cpp

@@ -654,6 +654,8 @@ int main(int argc, char* argv[]) {
 #else
     (void)shmName; // POSIX shm mapping (shm_open) added when porting off Windows
     auto* shmIPC = (SharedAudioBufferIPC*)std::calloc(1, sizeof(SharedAudioBufferIPC));
+    // VSTi crackle fix: 8-slot audio ring (see SharedMemoryIPC.h).
+    shmIPC->ringMask = 7;
     if (!shmIPC) return 1;
 #endif
 
@@ -754,9 +756,12 @@ int main(int argc, char* argv[]) {
     };
 
     // Render only [from, to) of the block — used by sample-accurate splitting.
+    // VSTi crackle fix: target the multi-slot ring slot for the block being
+    // built, so old blocks stay readable until the reader drains them.
     auto renderSegment = [&](uint32_t from, uint32_t to) {
         if (to <= from) return;
-        instruments.renderAll(shmIPC->masterLeft + from, shmIPC->masterRight + from, to - from);
+        uint32_t slot = shmIPC->bridgeWriteIndex & shmIPC->ringMask;
+        instruments.renderAll(shmIPC->ringLeft[slot] + from, shmIPC->ringRight[slot] + from, to - from);
     };
 
     double blockDurationMs = (double)block / sampleRate * 1000.0;

native_bridge/tests/shm_selfcheck.cpp

@@ -65,7 +65,18 @@ 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
+    // G4.5 ring: 11168 (G3.1 +heartbeat) + ringMask(4) + ringLeft(8192) + ringRight(8192)
+    // = 27556, rounded up to 8-byte struct alignment (u64 blockTimestamp) => 27560.
+    // Matches Rust size_of (src-tauri/src/shm.rs layout test).
+    assert(sizeof(SharedAudioBufferIPC) == 27560);
+
+    // 6. Audio ring round-trip (multi-slot, VSTi crackle fix)
+    ipc->ringMask = 7;
+    for (uint32_t i = 0; i < AUDIO_BLOCK_SIZE; ++i) ipc->ringLeft[3][i] = (float)i * 2.0f;
+    ipc->ringRight[3][0] = 42.0f;
+    assert(ipc->ringLeft[3][255] == 510.0f);
+    assert(ipc->ringRight[3][0] == 42.0f);
+    assert((ipc->bridgeWriteIndex & ipc->ringMask) < 8);
 
     std::printf("SHM self-check OK (sizeof struct = %zu bytes)\n", sizeof(SharedAudioBufferIPC));
     shm_close(h);