Image Trace: Ignore Color can leave out only the background

Squashed from #3952 onto main.

docs/image-trace-adjustments.md

@@ -51,6 +51,13 @@ traced fills and strokes take the library color without a link to its swatch. Th
 spot and tint swatches. Tints trace at the color shown by the tint swatch. Editing a swatch later
 does not recolor the trace. Colors from other swatch libraries are also unlinked.
 
+**Ignore Color, background only.** Ignore Color leaves out every area of the chosen color. With
+`"ignoreBackground": true` beside `"ignoreWhite": true` in the params of `imageTrace.make` or
+`imageTrace.makeAndExpand`, it leaves out only the areas of that color that reach the image's edge,
+joined through pixels above, below or to either side, and traces the areas of it that other colors
+enclose: black line work inside a panel stays when the black around the panel is left out. Flat Logo
+leaves out every white area whatever `ignoreBackground` is.
+
 **Document Library** traces with the library's colors as they are. Each pixel takes the library
 color nearest to it in lightness and hue, with a difference in hue or chroma counted three times
 as much as one in lightness, so a gray or a muted brown takes a black, white or other neutral

docs/target-app-parity.md

@@ -1104,6 +1104,7 @@ Moved from the ROADMAP's "Shipped so far" on 2026-10-10. Update it when a featur
 - **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)).
+- **Image Trace, Ignore Color for the background only:** `"ignoreBackground": true` with Ignore Color leaves out only the areas of the ignored color that reach the image's edge and traces the areas of it that other colors enclose, in Black and White, Grayscale and Color; code-drawn tests in `crates/trace` and `crates/engine`.
 - **Image Trace, Flat Logo working size:** Flat Logo traces at most 1024 px on a side; a larger decoded raster (the 64-megapixel copy for a bigger image) is averaged down in whole-number blocks first, and the trace's `warnings` name the size it was traced at and the image's own size, in place of the slower-trace and 64-megapixel notes; code-drawn 1024 and 1025 px tests in `crates/trace` and `crates/engine`.
 - **Image Trace, Flat Logo working size:** Flat Logo traces at most 1024 px on a side; a larger image is averaged down by a whole factor first, and the trace's `warnings` name the size it was traced at and the image's own size, in place of the slower-trace and 64-megapixel notes; code-drawn 1024 and 1025 px tests in `crates/trace` and `crates/engine`.
 - **Image Trace, Preview:** the Image Trace panel's Preview checkbox, before Trace, is on by default, and a setting change traces the selected Image Trace object again at once. Off, changes wait in the panel until Trace applies them as one undo step, or until Preview is turned on again. Changes stay with the object they were made for while another object is selected. When Preview is turned on with that object not selected, the object is traced with its changes when it is selected again. Preview is kept with the UI preferences, and Reset to Default leaves it. The `imageTrace.preview` command turns it on or off.