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/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);