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/editor_platform.cpp

@@ -219,9 +219,13 @@ LRESULT CALLBACK ReaSamplerEditor::wndProc(HWND hwnd, UINT msg, WPARAM wParam,
                 }
                 if (self->drag_ != DragKind::kNone) {
                     // A scrollbar drag + the processor-side deck knobs (preview velocity -2 /
-                    // voice count / master gain) mutate no parameter, so dragStartParams_ is
-                    // not a rollback target for them — reset drag state only. Every
-                    // parameter-editing drag restores the pre-grab snapshot.
+                    // voice count / master gain) mutate no params_ field, so dragStartParams_
+                    // is not a rollback target for them — reset drag state only. Every
+                    // params_-editing drag restores the pre-grab snapshot. Voice count and
+                    // master gain are pre-existing exceptions to that: both write straight
+                    // through on every move (editor voiceCount_ / processor masterGain_)
+                    // rather than through params_, so an abandoned drag leaves them at the
+                    // abandoned value indefinitely instead of rolling back.
                     const bool transient = self->drag_ == DragKind::kScrollThumb ||
                         (self->drag_ == DragKind::kDeckKnob &&
                          (self->dragParamId_ == -2 ||