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.

src/core/ui/CLAUDE.md

@@ -103,7 +103,7 @@ L7 sub-pass, 2026-07-27):
 - `card_meta` — pure card-metadata formatters: bars.beats.subdivisions and seconds.milliseconds; blank when the sample is unstamped.
 - `stroke_aa` — analytic antialiased thick-stroke COVERAGE (the shell blends it): `StrokeCanvas`, a reusable mask holding distance-to-polyline coverage MAX-accumulated across segments, plus `strokePolyline` / `strokeBounds` / `appendArc` / `rasterRowOffset` (the row-major offset
   math for a possibly bottom-up raster, pulled out of the shell's LICE blend so its flipped
-  branch is pinned by a host-free test). An arc is just a flattened polyline, so ONE path serves the knob arcs, the inner dial, the envelope polyline and both spline traces. Coverage is `clamp(halfWidth + 0.5 - distance, 0, 1)`, which makes perpendicular weight exactly `2·halfWidth` at every angle. **The guaranteed-opaque-core threshold is width ≥ 2 px, not any width above 1 px**: opacity needs `distance <= halfWidth - 0.5`, and the worst-case distance from a pixel centre to the centreline is 0.5, so a 1 px stroke (`halfWidth = 0.5`) has zero slack — its peak alpha modulates with the stroke's exact alignment to the pixel grid instead of pinning to 255 (the knob track arc and the mini curve-thumbnail trace are both 1 px and both live with this). Long segments are subdivided before rasterizing — EXACT, not an approximation (min-distance to a partition of a segment is min-distance to the whole), purely to keep each piece's bounding box tight, since one long diagonal's box has area O(len²).
+  branch is pinned by a host-free test). An arc is just a flattened polyline, so ONE path serves the knob arcs, the inner dial, the envelope polyline and both spline traces. Coverage is `clamp(halfWidth + 0.5 - distance, 0, 1)`, which makes perpendicular weight exactly `2·halfWidth` at every angle. **The guaranteed-opaque-core threshold is width ≥ 2 px, not any width above 1 px**: opacity needs `distance <= halfWidth - 0.5`, and the worst-case distance from a pixel centre to the centreline is 0.5, so a 1 px stroke (`halfWidth = 0.5`) has zero slack — its peak alpha modulates with the stroke's exact alignment to the pixel grid instead of pinning to 255 (Daniel's ruling, 2026-08-01: every stroker-drawn width on the editor is now >= 2 px for this reason — `testSubOpaqueCoreAtOnePixelWidth` in `tests/test_stroke_aa.cpp` still pins the 1 px case as a property of the stroker, independent of whether any surface ships at that width). Long segments are subdivided before rasterizing — EXACT, not an approximation (min-distance to a partition of a segment is min-distance to the whole), purely to keep each piece's bounding box tight, since one long diagonal's box has area O(len²).
 
 ## Gotchas