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.h

@@ -17,10 +17,10 @@ inline constexpr double kVelMax = 127.0;
 inline constexpr double kCurveYMax = 1.0;
 
 // The curve's Y range. UNIPOLAR [0,1] is a GAIN — the amp's domain, where the do-nothing
-// curve is flat at 1. BIPOLAR [-1,1] is a SIGNED modulation depth — the pitch and filter
+// curve is flat at 1. BIPOLAR [-1,1] is a SIGNED modulation shape — the pitch and filter
 // domains, where the do-nothing curve is flat at 0 and the sign picks the direction. A
-// bipolar curve is therefore both the shape and the amount: there is no separate depth
-// control behind it.
+// bipolar curve does not preclude a depth control beside it: the filter has one, and the two
+// compose multiplicatively (play_params.h).
 enum class CurveDomain { Unipolar, Bipolar };
 
 constexpr double curveYMin(CurveDomain d) { return d == CurveDomain::Bipolar ? -1.0 : 0.0; }
@@ -46,11 +46,7 @@ inline constexpr int kCurveNodeGrabRadius = 6;
 // overshoots a segment's value range. For collinear knots the tangents reduce to the secant
 // slope, so the spline reproduces linear()'s straight line to within ~1e-15. The two endpoints
 // (velocity 0 and 127) are load-bearing: they keep eval total over the domain and are never
-// deletable. eval is HOMOGENEOUS in y — scaling every knot's value by k scales the whole curve
-// by k TO WITHIN DOUBLE ROUNDING (the Hermite basis and the Fritsch-Carlson tangent are exactly
-// degree-1 homogeneous in real arithmetic; `fl(k*b) - fl(k*a)` isn't bit-identical to
-// `k*(b-a)`), which is what lets the codec (component_state_io.h's v12 pre-lift) fold a retired
-// depth control into stored knots and still sound identical.
+// deletable.
 class VelocityCurve {
 public:
     // flat() (endpoints (0,1)/(127,1), every velocity -> unity) is the unipolar default — see