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/shell/instrument/editor_stroke.h

@@ -0,0 +1,50 @@
+// editor_stroke.h — the editor's LICE side of the analytic stroker: build a coverage mask with
+// the pure `core/ui/stroke_aa` module, blend it into the bitmap ONCE.
+//
+// Every radial and spline stroke on the editor goes through here. LICE's own primitives cannot
+// serve these two shapes: LICE_Arc rasterizes a whole circle clipped per 90-degree box and splits
+// one unit of ink across two pixels by the radius's fractional part (so a stacked-radii arc never
+// reaches an opaque core), and LICE_ThickFLine lays its width along the MINOR axis (so a curve's
+// perpendicular weight falls off as cos(theta) and thins at every diagonal).
+
+#pragma once
+
+#ifdef _WIN32
+
+#include <cstddef>
+#include <vector>
+
+#include "lice/lice.h"
+#include "wdltypes.h"
+
+#include "core/ui/rect.h"
+#include "core/ui/stroke_aa.h"
+
+namespace reasampler::vst {
+
+// Blends a polyline as one antialiased stroke of `widthPx`. Clipped to the bitmap. Takes a raw
+// span so a short stroke (a knob needle) can pass a stack array and cost no allocation.
+void strokePolylineAA(LICE_IBitmap* bmp, const ui::StrokePoint* pts, std::size_t count,
+                      float widthPx, LICE_pixel color, float alpha = 1.0f);
+
+inline void strokePolylineAA(LICE_IBitmap* bmp, const std::vector<ui::StrokePoint>& pts,
+                             float widthPx, LICE_pixel color, float alpha = 1.0f) {
+    strokePolylineAA(bmp, pts.data(), pts.size(), widthPx, color, alpha);
+}
+
+// A single antialiased segment — the two-point polyline, spelled out for the call sites that
+// have exactly two endpoints.
+inline void strokeLineAA(LICE_IBitmap* bmp, float x0, float y0, float x1, float y1, float widthPx,
+                         LICE_pixel color, float alpha = 1.0f) {
+    const ui::StrokePoint pts[2] = {{x0, y0}, {x1, y1}};
+    strokePolylineAA(bmp, pts, 2, widthPx, color, alpha);
+}
+
+// Blends a circular arc as one antialiased stroke of `widthPx`. Angles are radians in LICE's
+// convention (0 = 12 o'clock, increasing clockwise), matching what LICE_Arc took.
+void strokeArcAA(LICE_IBitmap* bmp, float cx, float cy, float radius, float startRad, float endRad,
+                 float widthPx, LICE_pixel color, float alpha = 1.0f);
+
+}  // namespace reasampler::vst
+
+#endif  // _WIN32