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.