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/reasampler_processor.h

@@ -239,6 +239,11 @@ private:
     // Both live_ and draining_ observe this same block — a block owned by a snapshot would
     // leave the drain's still-sounding voices deaf to the knob under them.
     instrument::engine::LiveParams liveParams_;
+    // Serializes liveParams_.publish's two writer sites (reloadInstrument, publishLiveParams)
+    // only — separate from reloadMutex_ so a knob drag's publish never blocks behind a
+    // reload's WAV decode. The audio thread never takes this; process() only reads via
+    // LiveParams::read's lock-free seqlock retry.
+    std::mutex livePublishMutex_;
     // The rate the loaded capture was decoded/built at, so a live republish resolves the
     // stored wall-clock seconds to exactly the frames the built SampleData carries. 0 = nothing
     // built yet.