instrument: snap live params onto a fresh voice, roll a live drag back on capture loss, serialize the seqlock's two writers

src/shell/instrument/processor_state.cpp

@@ -153,6 +153,13 @@ void ReaSamplerProcessor::setInstrumentParams(const InstrumentParams& params) {
 }
 
 void ReaSamplerProcessor::publishLiveParams() {
+    // reloadMutex_ enforces the block's SINGLE-WRITER contract (live_params.h), not the
+    // reload's slot bookkeeping: reloadInstrument publishes the block too, and two concurrent
+    // seqlock writers can leave the generation even mid-write, which a reader would accept as
+    // a coherent — but torn — block. The audio thread never takes this mutex, so the cost is
+    // an off-thread wait behind a reload. Lock order matches reloadInstrument's
+    // (reloadMutex_ then paramsMutex_, taken by instrumentParams below).
+    std::lock_guard<std::mutex> lock(reloadMutex_);
     const int rate = builtSampleRate_.load(std::memory_order_relaxed);
     if (rate <= 0) return;
     liveParams_.publish(