Image Trace: a nearly flat dark ground keeps one color in a Limited palette

The Limited palette's median cut ranks cuts by pixel-weighted gain. A
large near-black ground with faint noise therefore took two colors less
than 0.02 apart in Oklab, and the trace drew blotches of one over the
other.

With 9 to 16 colors, a cut whose two sides both have a dark gray mean
(Oklab lightness under 0.30, chroma under 0.03) and lie less than 0.03
apart now has its gain scaled by (distance / 0.03)^2 while the cut is
chosen. The ground keeps one color, and the color it would have taken
goes to another cut. Document Library without swatches follows Limited.
Other color counts, Full Tone and Automatic choose the same cuts as
before.

docs/target-app-parity.md

@@ -1371,6 +1371,7 @@ Moved from the ROADMAP's "Shipped so far" on 2026-10-10. Update it when a featur
 - **Image Trace, Document Library colors:** with Document Swatches, traced fills and strokes use the displayed colors of process, global, spot and tint swatches without links, so editing a swatch leaves the trace unchanged; swatch-library colors are also unlinked; code-drawn tests in `crates/engine`.
 - **Image Trace, flattened slivers:** in a Color, Grayscale or Black and White trace with Fills, a traced region whose outline straightens onto one line (a single pixel, or a one-pixel row along an edge, its pieces shared with the regions beside it) is left out instead of becoming a selectable path with no area, which paints nothing. Snap Curves To Lines straightens more outlines, so it leaves out more of these. Without Fills, the region's 1 px stroke draws it, and it is kept. Code-drawn tests in `crates/trace`.
 - **Image Trace, Auto Color rare colours:** the Automatic palette keeps colours too rare for its area count when they are distinct (Oklab) and drawn as coherent regions, at most six, protected from mixed-colour merging and Noise; code-drawn stem, highlight, low-contrast and scattered-noise tests (see [quality notes](image-trace-quality.md#auto-color-keeps-rare-colours)).
+- **Image Trace, nearly flat dark grounds:** with a Limited palette of 9 to 16 colors (16 Colors, and Document Library without swatches), by VectorCraft's own rule, a cut whose two sides both have a dark gray mean (Oklab lightness under 0.30, chroma under 0.03) and lie less than 0.03 apart in Oklab has its gain scaled by (distance / 0.03)². A near-black ground with faint noise then traces in one color, and the color it would have taken goes to another cut. Code-drawn tests `a_nearly_flat_dark_ground_keeps_one_color`, `only_a_limited_palette_of_nine_to_sixteen_colors_weighs_alike_sides` and `a_dark_colored_figure_on_a_nearly_flat_dark_ground_keeps_its_color` in `crates/trace`.
 - **Image Trace, Document Library keeps neutral tones neutral:** each pixel takes the library color nearest in Oklab with differences in hue and chroma counted three times, so a gray or a muted brown traces as a neutral library color instead of a saturated one of similar lightness; a code-drawn test of gray and brown patches with a black, light and blue library in `crates/trace`.
 - **Image Trace, source image on hold:** while the eye button next to the Image Trace panel's View is held down, the canvas shows the selected Image Trace objects' source image whatever their View, and shows their View again on release, with no change to the document or the undo history.
 - **Image Trace, large images:** images over 1500 px on a side trace with a warning in the result and the status bar; the anchor limit is 8,000,000 (2,000,000 on the web), measured at about 90 bytes an anchor (see [quality notes](image-trace-quality.md#large-images)).