PDF export: a Perceptual gradient that mixes spot and process inks keeps its inks

A Perceptual linear or radial gradient that mixes spot colors with each
other or with CMYK or gray stops was written as an image in process
colors. It is now written as a DeviceN shading of its inks, with the
stops on its Perceptual curve: at each one, the inks of the two stops on
either side are mixed at the weight where the two colors mixed in
display RGB have the OKLab lightness of the color on the curve.

- Gradient::ink_mixes gives the curve's stops as weights between stops,
  under the same 4,096-stop limit as ink_tints.
- Perceptual gradients that mix a spot color with RGB or Lab stops,
  dithered fills and freeform gradients stay images, and the warning
  now names those cases.

docs/target-app-parity.md

@@ -1278,7 +1278,7 @@ applies to fills). Tests: `color::gradient::tests` (perceptual midpoint, expansi
 `pdf::tests::dithered_perceptual_gradients_export_as_images_with_noise_on_the_fill` and the Gradient panel's
 `the_method_menu_makes_the_gradient_perceptual`. Scores and hours unchanged.
 
-PDF export writes a Perceptual linear or radial gradient whose stops are tints of one spot color (and paper white) as a Separation shading in that ink instead of an image, so the ink keeps its plate ([swatches](https://helpx.adobe.com/illustrator/using/using-creating-swatches.html): "Gradient colors can be assigned as CMYK process colors, RGB colors, or a Spot color."; [separations](https://helpx.adobe.com/illustrator/using/printing-color-separations.html): "a separate plate is created for each spot color"). Its tints follow the curve's stops, each the tint with the OKLab lightness of the color there (VectorCraft's own rule). Dithered fills, freeform gradients and Perceptual gradients that mix a spot color with other colors are still images, and the export warns that their spot colors are written in process colors. Tests: `color::gradient::tests::ink_tints_follow_the_perceptual_curve`, `engine::tests::linked_stops::perceptual_spot_gradients_export_as_separation_shadings` and `spot_gradients_drawn_as_images_warn`.
+PDF export writes a Perceptual linear or radial gradient whose stops are tints of one spot color (and paper white) as a Separation shading in that ink instead of an image, so the ink keeps its plate ([swatches](https://helpx.adobe.com/illustrator/using/using-creating-swatches.html): "Gradient colors can be assigned as CMYK process colors, RGB colors, or a Spot color."; [separations](https://helpx.adobe.com/illustrator/using/printing-color-separations.html): "a separate plate is created for each spot color"). Its tints follow the curve's stops, each the tint with the OKLab lightness of the color there (VectorCraft's own rule). A Perceptual gradient that mixes spot colors with each other or with CMYK or gray stops is written as a DeviceN shading of its inks: at each stop on the curve, the inks of the two stops on either side are mixed at the weight where the two colors mixed in display RGB have the OKLab lightness of the color there (VectorCraft's own rule). Dithered fills, freeform gradients and Perceptual gradients that mix a spot color with RGB or Lab stops are still images, and the export warns that their spot colors are written in process colors. Tests: `color::gradient::tests::ink_tints_follow_the_perceptual_curve` and `ink_mixes_follow_the_perceptual_curve`, `engine::tests::linked_stops::perceptual_spot_gradients_export_as_separation_shadings`, `perceptual_spot_gradients_mixed_with_other_inks_export_as_device_n_shadings` and `spot_gradients_drawn_as_images_warn`.
 
 ## Artwork Cleanup add-on (2026-10-30)