instrument: the filter's velocity depth knob returns and multiplies the bipolar curve; the pre-v12 lift is a pure domain re-tag

src/core/instrument/engine/live_params.h

@@ -34,6 +34,10 @@ inline constexpr double kLiveRampSeconds = 0.020;
 struct LiveValues {
     filter::FilterSettings filterSettings{};
     double       filterModAmount = 0.0;
+    // The DEPTH scaling the velocity curve, not the curve's value: the note's velocity is
+    // latched, its depth is a control, exactly as filterKeyTrack is a control over a latched
+    // note number.
+    double       filterVelAmount = 0.0;
     double       filterKeyTrack = 0.0;
     AdsrParams   filterEnv{};
     AhdParams    filterAhd{};