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/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;