PITCH/RATE deck: Rate and Pitch knobs compounded into one read increment, on a three-state commit predicate and payload v16

src/core/instrument/engine/play_params.h

@@ -158,6 +158,15 @@ struct PlayParams {
     TriggerParams  trigger;   // Trigger play span
     AhdParams      trigAhd;   // Trigger amp
     PitchEngine    pitchEngine = PitchEngine::Varispeed;
+    // Playback RATE, as source frames consumed per output frame. Under Varispeed it is one more
+    // factor of the read increment, so it moves pitch and duration together; under Preserve it
+    // drives duration alone and the shifter holds the pitch. Latched at note-on either way (the
+    // loop fold and the contour scale it composes with are both note-on folds), and clamped by
+    // the stretcher's own clampStretchRate — never here. 1.0 is the bare engine, bit for bit.
+    double         playRate = 1.0;
+    // A baseline pitch offset in semitones, folded into the note's ratio beside key-tracking and
+    // the velocity->pitch transpose. Live on a sounding voice under both engines.
+    double         pitchOffsetSemitones = 0.0;
     PitchEnvParams pitchEnv;
     // Velocity -> pitch offset, scaled by kVelocityPitchRangeSemitones. Bipolar and flat at 0
     // by default, so it transposes nothing until a curve is drawn. Folded into the voice's