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/velocity_curve.cpp

@@ -97,8 +97,7 @@ namespace {
 // Fritsch-Carlson monotone-cubic tangent: a sign change (or flat) neighbour is a local extremum,
 // so the tangent pins to 0 to avoid overshoot; otherwise the weighted-harmonic-mean tangent,
 // which for collinear knots (dPrev==dNext) reduces exactly to the shared secant — this is what
-// makes the spline reproduce a straight line for linear()-style input. Homogeneous of degree 1
-// in the secants, which is what makes eval homogeneous in y (see the header).
+// makes the spline reproduce a straight line for linear()-style input.
 double fritschCarlsonTangent(double dPrev, double dNext, double spanPrev, double spanNext) {
     if (dPrev * dNext <= 0.0) return 0.0;
     const double w1 = 2.0 * spanNext + spanPrev;