fix: enlarge every sub-2px stroker width to 2px, per Daniel's ruling

Knob track arc, inner-dial needle, and mini velocity-trace all sat below the analytic stroker's opaque-core floor. Raised to 2px; updated visual-design-language.md and core/ui/CLAUDE.md to match; kept the 1px-behavior test, corrected its stale comment.

tests/test_stroke_aa.cpp

@@ -91,8 +91,9 @@ static void testSubOpaqueCoreAtOnePixelWidth() {
     // Below the >= 2 px opaque-core threshold: a 1 px stroke (halfWidth = 0.5) has zero slack
     // against the 0.5 px worst-case pixel-centre distance (core/ui/CLAUDE.md), so peak alpha
     // tracks the stroke's alignment to the pixel grid instead of reaching 255 everywhere. Pin
-    // both ends of that modulation — this is the knob track arc's and the mini curve-trace's
-    // actual behaviour, not a hypothetical.
+    // both ends of that modulation — no shipping surface draws at 1 px anymore (Daniel's
+    // ruling, 2026-08-01), but this stays a real property of the stroker worth guarding
+    // against reintroducing.
     const StrokePoint onRowCentre[2] = {{20.0f, 100.5f}, {220.0f, 100.5f}};  // centred on row 100
     StrokeCanvas aligned;
     strokePolyline(aligned, onRowCentre, 2, 0.5f, kBig);