docs: record Θ-W7, the arc-and-spline antialiasing fix, and file the scaled-fallback deferral

docs/COMPLETED.md

@@ -613,3 +613,59 @@ reset semantics unit-testable; `editor_controls.cpp` dropped 469→293 lines.
 All visual outcomes remain **pending Daniel's by-eye sign-off on `dev`** — sizes,
 arc weight, and whether the waveform stroke improves or thickens the docked panel.
 Not recorded as accepted.
+
+### Θ-W7-T1 — arc-and-spline-aa
+
+Two defects Daniel found by eye once Θ-W6-T1's antialiasing pass shipped — diagnosing
+both corrected the initial reading of each.
+
+- **Arcs never reached opacity.** `LICE_Arc` rasterizes a whole circle clipped per 90°
+  chunk and splits ink across two pixels by the fractional part of the radius;
+  `rOuter = radius - 0.5f` is half-integer, so no pixel in the ring was ever opaque —
+  measured peak alpha 138/255. The three stacked radii also did not tile: spacing
+  dilates from 1.0 px to 1.41 px at 45°, leaving partial-coverage holes. It read as
+  fuzz, but it was a stroke that never fully inked.
+- **Splines were fully aliased, not gapped.** The apparent dotting was not missing
+  ink: `LICE_ThickFLine` steps the major axis and structurally cannot gap. The paint
+  loop passed **integer** `cx`/`cy`, so LICE had no sub-pixel position to interpolate
+  — every pixel was full or empty with no AA fringe, and integer `cy` quantized the
+  slope into an alternating 1/2 px staircase that reads as beading at 100%.
+- **The cheap fix was rejected.** Float endpoints plus `LICE_ThickFLine` fixes
+  opacity and the staircase, but `ThickFLine` lays width along the *minor* axis, so
+  perpendicular weight is `wid·cosθ` — a measured 42% ripple dipping at every 45°
+  diagonal.
+- **What landed:** one pure analytic thick-stroke rasterizer. Coverage is
+  distance-to-polyline, accumulated with `max()` into a scratch buffer and blended
+  **once** — the single blend is what structurally prevents the compositing fringe
+  build-up behind the first defect. An arc is just a polyline, so one code path
+  replaces the stacked arcs, both spline traces, and the two needles. Pure coverage
+  math in a new `core/ui/stroke_aa`; the blend loop in a new
+  `shell/instrument/editor_stroke`. `shell/panel/draw_kit` was deliberately **not**
+  touched, keeping the docked bank panel and browse cards entirely out of the blast
+  radius.
+- **Measured, before → after:** arc peak alpha 138/255 → 255/255; arc perpendicular
+  weight 1.62–3.24 px (67% ripple) → 2.95–3.11 px (5%); spline weight 1.41–2.00 px
+  (29%) → 1.95–2.01 px (3%). Cost: **+0.09 ms per full editor repaint** (30 arcs
+  0.113 → 0.169 ms; 500 px contour 0.013 → 0.047 ms), a knowing regression on an
+  interaction-driven surface, measured in Release against real LICE in an
+  uncommitted harness.
+- **`velocity_curve` gained `subpixelFromPoint`** — sub-pixel y was unavoidable since
+  integer `cy` was the root cause. The existing integer map now *rounds* the new
+  float map rather than forking a second formula, so hit-testing is unchanged.
+- **Daniel then ruled that every sub-2 px stroker width be enlarged**, because the
+  stroker can only guarantee an opaque core at width >= 2 px (an opaque pixel needs
+  `d <= halfWidth − 0.5`, and the worst-case pixel-centre-to-centreline distance is
+  0.5). The knob track arc, the inner-dial needle, and the deck's mini velocity trace
+  all moved 1.0 → 2.0 px. A test pinning the sub-opaque behaviour at 1 px was kept as
+  a guard against reintroduction.
+- **The audit's method was the root failure, not its output.**
+  `docs/product/visual-design-language.md` §8 had claimed stacked 1 px `LICE_Arc`
+  calls "keep every ring antialiased" — false. The Θ-W6 audit verified *which
+  primitive was called* rather than *what it rasterized*, which is how both surfaces
+  were signed off clean while never producing an opaque pixel. That sentence is
+  deleted, the rows are re-dispositioned with measurements, and the methodological
+  lesson is recorded in §8 as a standing blockquote.
+
+All visual outcomes remain **pending Daniel's by-eye sign-off on `dev`** — nothing was
+verified in a live REAPER window; all measurement was against an offscreen bitmap in a
+standalone harness. Not recorded as accepted.