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_reload.cpp

@@ -154,7 +154,12 @@ std::string ReaSamplerProcessor::reloadInstrument() {
                 // frames the build resolved and a note-on with a live block sounds identical
                 // to one without.
                 sample.live = &liveParams_;
-                liveParams_.publish(instrument::engine::foldLive(sample.play));
+                {
+                    // reloadMutex_ (held for this whole function) nests livePublishMutex_ here;
+                    // publishLiveParams never holds reloadMutex_, so this is the only nesting.
+                    std::lock_guard<std::mutex> lp(livePublishMutex_);
+                    liveParams_.publish(instrument::engine::foldLive(sample.play));
+                }
                 builtSampleRate_.store(sample.sampleRate, std::memory_order_relaxed);
                 resolvedId = selId;  // the concrete pick that resolved
             }