Swatches: a gradient stop unlinked from its spot swatch also drops its plates

A gradient stop that loses its link to a spot swatch kept the process
plates it printed with that ink, so it carried a DeviceN mix with no
ink. Paint::map_links, which deleting a swatch, recoloring and color
adjustments go through, now clears a stop's plates whenever it leaves
the stop without a link, as GradientStop::set_color(color, None)
already does. Library gradients and library character styles linked
to a swatch the document lacks unlink their stops the same way.

Docs: docs/target-app-parity.md.

docs/target-app-parity.md

@@ -1507,6 +1507,7 @@ Moved from the ROADMAP's "Shipped so far" on 2026-10-10. Update it when a featur
   - **Sample Color from Screen:** an eyedropper button in the Color Picker and `ui.sampleScreenColor` (which can be given a key in Keyboard Shortcuts) pick a color anywhere on the screen with the system's sampler: `NSColorSampler` on macOS, the XDG desktop portal's [PickColor](https://flatpak.github.io/xdg-desktop-portal/docs/doc-org.freedesktop.portal.Screenshot.html) on Linux, and the EyeDropper API in browsers that have it. The color becomes the fill or the stroke as one undo step, or the Color Picker's color, converted from display sRGB into the working RGB space. On Linux, a session without a portal that has PickColor, a portal that does not return from the call within 25 seconds (VectorCraft's own limit) or leaves the bus before it answers, and a portal that ends the request another way each show a status message and end the wait. Windows, the BSDs and other browsers have no sampler yet; the button is gray there.
   - **Stroke visibility (#1253):** adding paint to a previously unpainted zero-width stroke restores the existing 1 pt default (`PaintDefaults::default`, `Appearance::set_stroke`), for the selected appearance item, the new-art defaults and Swap Fill/Stroke. Positive widths are kept; recoloring a painted zero-width stroke keeps its explicit zero. A real Eyedropper-to-new-shape sequence reproduces the zero-width inheritance. Five engine regressions cover creation, defaults, targeted items, preserved widths and one-step Undo/Redo (`crates/engine/tests/it/stroke_visibility.rs`).
   - **Gradients:** lossless, validated gradient params; gradients follow rotate, reflect, shear, non-uniform scale and distortions; the Gradient tool edits the fill or stroke of the active proxy, type included; an interactive on-canvas annotator (move, length/angle, rotate, add/move/delete/duplicate stops, midpoints, a stop popover, Delete and arrow keys). The Gradient panel has the Fill/Stroke proxy, a gradient-swatch dropdown with Save to Swatches, midpoint locations, Alt-drag to copy or swap stops, swatch drops on the ramp and a stop eyedropper; copied appearances and graphic styles place gradients on each target's own bounds; gradient handles snap and honour Constrain Angle. Gradient stops stay linked to global and spot swatches. Freeform gradients (points and lines) are edited on the canvas; radial gradients have an extent ellipse, an aspect handle and an off-centre focal point (SVG `fx`/`fy`, PDF two-point radials). A stroke's gradient runs within, along or across it. Object ▸ Expand turns gradients into N objects or a gradient mesh.
+    A gradient stop that loses its link to a spot swatch (the swatch deleted, a recolor or adjustment that unlinks it, a library gradient, or a library character style linked to a swatch the document lacks) keeps the color it shows at full tint and drops the process plates it printed with that ink, as a stop given a plain color does; this is VectorCraft's own rule. Evidence: `tests::mapping_links_drops_the_process_plates_of_unlinked_stops` in `crates/color`, `tests::linked_stops::deleting_a_spot_swatch_takes_the_process_plates_of_the_stops_it_unlinks` and `tests::library::library_stops_unlinked_from_their_swatch_keep_no_process_plates` in `crates/engine`.
   - The Gradient panel's Delete Stop sits at the right end of the ramp, level with it, so a panel too short to show the stop fields still shows it; it deletes the selected stop (never below two), as dragging a stop down off the ramp does (#5403).
   - Conic gradients, VectorCraft's own gradient type: the stops sweep once around the center from the vector, clockwise on the page (a negative aspect, or a reflection, turns the sweep the other way), with a Conic button in the Gradient panel's Type row and the Gradient tool's extent ellipse and aspect handle; they follow transforms, Expand into wedges or a mesh, open from `.af` files, and export to PDF as a function-based shading that PDF import reads back as a conic gradient; SVG, EPS and legacy `.ai` files get solid wedges, and CSS a `conic-gradient`.
   - Diamond gradients, VectorCraft's own gradient type: the stops run from the center out to a square with a corner at the end of the vector (the aspect stretches it into a rhombus), with a Diamond button in the Gradient panel's Type row and the Gradient tool's square and aspect handle; they follow transforms, Expand into nested squares or a mesh, and export to PDF, SVG, EPS and legacy `.ai` as four linear gradients, one for each quarter of the square (an image in PDF and SVG for translucent stops, a dithered fill, the Perceptual method other than spot color shadings in PDF, or a square with no area), and on type characters in SVG as a radial gradient; PDF import reads a function-based shading whose color is the same along each square about its origin as a diamond gradient.