instrument: narrow the live-param publish lock to its own mutex, off the reload's decode

Knob-drag publishes no longer block behind a full reload's WAV decode — a dedicated livePublishMutex_ replaces reloadMutex_ for the seqlock's single-writer contract. Also fixes an editor comment overclaim and two doc restatements.

src/shell/instrument/processor_state.cpp

@@ -153,17 +153,14 @@ 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(
-        instrument::engine::foldLive(resolvePlay(instrumentParams().play, rate)));
+    const instrument::engine::LiveValues block =
+        instrument::engine::foldLive(resolvePlay(instrumentParams().play, rate));
+    // livePublishMutex_ enforces the seqlock's single-writer contract (live_params.h) against
+    // reloadInstrument's publish — held for the publish call only, not the fold above.
+    std::lock_guard<std::mutex> lock(livePublishMutex_);
+    liveParams_.publish(block);
 }
 
 SampleRefs ReaSamplerProcessor::sampleRefs() {