Swatches: Add Used Colors and Select All Unused look colors up in hash sets

swatch.addUsedColors and swatch.unused searched lists inside loops over
colors and swatches, checking each color against every color or swatch
before it. They now keep the same ordered lists and use hash sets and
maps for membership and lookup. A color's key is its model and the bits
of its components with -0.0 read as 0.0, so the key agrees with `==`; a
color with a NaN component matches nothing, as before.

Add Used Colors names its new swatches through
Document::free_swatch_names, which collects the taken swatch and color
group names once and gives the same names free_swatch_name gives when the
swatches are added one at a time. The added swatches, their names, order
and links, the undo step, and the swatches Select All Unused lists are
unchanged.

docs/target-app-parity.md

@@ -1119,6 +1119,7 @@ Moved from the ROADMAP's "Shipped so far" on 2026-10-10. Update it when a featur
   - **CSS swatch libraries:** fractional linear and radial gradient stop positions are preserved, so nearby stops such as 12.5% and 12.890625% remain distinct instead of collapsing to 13%. Reference: [CSS Images 3 color stops](https://www.w3.org/TR/css-images-3/#color-stop-syntax). The original synthetic regression is `palette_io::tests::css_preserves_fractional_gradient_stop_positions` in `crates/color/src/palette_io.rs`. Scores and hours are unchanged.
   - **Swatches:** names follow the colour model and are unique; Swatch Options (process/spot, Global, Gray/RGB/HSB/CMYK/Web, live preview); editing a global or spot swatch recolours every linked fill, stroke and text run in one undo step; multi-select and delete with confirmation; New Swatch and New Color Group dialogs (a group from the selected artwork's colours); swatches in colour groups convert, separate and count like any other. The Swatches panel selects ranges and whole groups, has a find field, and drags swatches to reorder, regroup or paint art; Add Used Colors, Select All Unused, Merge, Ungroup and Sort by Kind; a CMYK document keeps applied colours in CMYK. Swatch libraries: nine generated colour libraries, five gradient libraries and a pattern library (Geometric Patterns, whose swatches bring their patterns) in a library panel, saved as `.vcswatches`, `.gpl`, swatch exchange (`.ase`) or CSS, and loaded from `.vcswatches`, `.gpl` and `.ase` files (`.ase` both ways: RGB, CMYK, Lab and Gray; global, spot and process; groups) and from color books (`.acb`, read only: the RGB, CMYK or Lab colors of a book such as a Pantone book a user owns, named with its prefix and suffix, spot unless it says process, #831); `swatch.library.copyToUser` copies a loaded library into the User Defined folder without replacing a file, and a file with the same extension and bytes as a User Defined library opens as that library. Tints of global and spot colours ("Name 40%") stay linked and separate as a percentage of their plate. Spot colours can be defined in Lab (Spot Colors options). A built-in [Registration] swatch prints on every plate.
     Editing, renaming, deleting or merging a global swatch also changes the colors linked to it in graph designs, the art of brushes and the tile that pattern editing's Cancel puts back, in the same undo step as the rest of the art; Default Swatches with replace unlinks them there too.
+    Add Used Colors (`swatch.addUsedColors`) and Select All Unused (`swatch.unused`) look colors, swatch names, links and tints up in hash sets, so their time grows in step with the number of colors in a document rather than with its square. The swatches Add Used Colors adds keep their names, order and links, and Select All Unused lists the same swatches. Evidence: `add_used_colors_adds_each_new_color_once_with_free_names`, `add_used_colors_global_links_every_paint_of_a_new_color` and `unused_matches_links_tints_colors_gradients_and_patterns_exactly` in `crates/engine/src/tests/swatchcmds.rs`, `free_swatch_names_match_naming_each_swatch_as_it_is_added` in `crates/doc/src/swatches.rs`.
   - **Colorized images (#1466):** a fill color applied to a selected grayscale or 1-bit image (`paint.setFill`) colorizes it with black, white, a process color or a spot color, and refuses a process color when the image has an alpha channel ([Convert color modes](https://web.archive.org/web/20251220223850/https://helpx.adobe.com/illustrator/desktop/manage-colors/select-and-adjust-colors/convert-color-modes.html), "Colorize Grayscale or 1-bit images"); black and white are accepted there too. The canvas draws each pixel in the color as far as the pixel is dark, with white pixels transparent; PDF export writes the image in the color (a spot color as its ink, Registration in every ink) with the image as its soft mask; Separations Preview and printed separations show it on its color's plates; native files keep the color. SVG writes the fill color through a coverage mask (or a colorized PNG in Tiny 1.2); EPS writes binary coverage as an image mask in the color's ink, and a grayscale image as ink samples scaled by coverage, with white pixels left out at Level 3 and gray midtones written over white.
     The prompt for deleting several swatches and groups uses the interface language's plural form for the count. Evidence: `deleting_several_swatches_uses_each_count_form` in `crates/ui-egui/src/panels/swatches/tests.rs`.
   - **Colorized images (#1466):** a fill color applied to a selected grayscale or 1-bit image (`paint.setFill`) colorizes it with black, white, a process color or a spot color, and refuses a process color when the image has an alpha channel ([Convert color modes](https://web.archive.org/web/20251220223850/https://helpx.adobe.com/illustrator/desktop/manage-colors/select-and-adjust-colors/convert-color-modes.html), "Colorize Grayscale or 1-bit images"); black and white are accepted there too. The canvas draws each pixel in the color as far as the pixel is dark, with white pixels transparent; PDF export writes the image in the color (a spot color as its ink, Registration in every ink) with the image as its soft mask; Separations Preview and printed separations show it on its color's plates; native files keep the color. EPS and SVG write the image's own pixels.