fix: stroke arcs and splines analytically — opaque core, angle-independent weight

LICE_Arc never reaches opacity and ThickFLine's width is minor-axis. One
distance-to-polyline coverage mask, blended once, replaces both.

src/core/instrument/engine/velocity_curve.h

@@ -182,6 +182,16 @@ public:
     };
     CurvePixel pixelFromPoint(const Box& box, const VelocityPoint& p) const;
 
+    // The SAME mapping before rounding: pixelFromPoint IS this, rounded, so a sub-pixel trace and
+    // an integer hit-test cannot drift. An antialiased stroke needs the fraction — quantizing y to
+    // a whole pixel forces the slope into alternating 1/2-px steps, and that beat-frequency
+    // staircase is what read as a dotted line where a contour steepened.
+    struct CurvePixelF {
+        double x = 0.0;
+        double y = 0.0;
+    };
+    CurvePixelF subpixelFromPoint(const Box& box, const VelocityPoint& p) const;
+
     // Exact inverse of pixelFromPoint (within the one-pixel quantum) — where an empty-space
     // click lands as a new point. Degenerate box: zero-width reads velocity 0; height <= 1
     // reads the domain's max (the top row is what a collapsed box draws).