PDF export: a gradient that mixes a spot color with process colors keeps the spot ink

A linear or radial gradient whose stops mix a spot color with process
colors, or two spot colors, was written in process colors with a
warning. It is now written as a DeviceN shading of the process inks
its stops use and its spot inks, so each spot ink separates onto its
own plate. An off-center midpoint becomes a stop with the inks halfway
between its two stops. A gradient that mixes a spot color with RGB or
Lab colors is still written without the spot ink, with a warning that
says so.

docs/target-app-parity.md

@@ -393,7 +393,7 @@ about 1–1.5 agent-hours each, so the other rows (estimated on the older scale)
 | Selection, transform & align tools | 6 | 85% | Direct Selection segment click/Delete corrected (#1278); remaining: Free Transform/Scale/Reflect along a rotated box, Global Edit of similar groups, transform nuances (the rotated persistent bounding box is done) | 4–6 |
 | Drawing tools | 7 | 86% | rough-square Shaper recognition and orientation steps corrected; editable Shaper Groups landed; Pencil: Alt key toggles to Smooth Tool (#2001); remaining: Shaper construction widget/gesture refinements, pencil and the remaining pen modifier nuances, Blob Brush Merge Only with Selection, Touch Type, Live Corners' Relative rounding | 7–12 |
 | Path operations, Pathfinder, Shape Builder, Live Paint | 5 | 86% | Live Paint gap options, Shape Builder gap detection, options (Tool Options dialog, clicking a stroke in merge mode) and Shift-drag marquee | 3–6 |
-| Colour, swatches, gradients, patterns, mesh, recolor | 7 | 96% | freeform and mixed spot/process gradients in SVG/PDF (exported as stops or process colours), pattern fills in Expand | 2–4; Eyedropper source reads regression-tested (#1251) |
+| Colour, swatches, gradients, patterns, mesh, recolor | 7 | 96% | freeform gradients in SVG/PDF and mixed spot/process gradients in SVG (exported as stops; PDF export keeps the spot inks in a DeviceN shading), pattern fills in Expand | 2–4; Eyedropper source reads regression-tested (#1251) |
 | Strokes, brushes, width profiles | 5 | 90% | the Art brush's Overlap, auto-generated Pattern brush corners and new tile art in Pattern Brush Options (Brush Options for every brush type done), more built-in brush libraries (four generated in code are done), pen pressure on macOS (the windowing layer reports none there), brushes driven by pen tilt, bearing and barrel rotation (6D art pens) | 8–15 |
 | Appearance, transparency, graphic styles, masks | 5 | 98% | raster effect reach in preview bounds (flattener presets apply in PDF, EPS and print) | 1–2 |
 | Live vector effects | 5 | 85% | Outline Object, Pathfinder Hard/Soft Mix and Trap (since done: #3061, #3063, #3785), SVG Filters, Transform Patterns and Scale Strokes & Effects on a fill's or stroke's own Transform effect | 6–10 |
@@ -1027,6 +1027,7 @@ Moved from the ROADMAP's "Shipped so far" on 2026-10-10. Update it when a featur
   - PDF export and print composition use the optional shared guarded writer ([pdf-backend.md](pdf-backend.md)); app drawing and output policy and desktop printer submission remain in their existing layers. The shared reader keeps the app-owned import adapter; synthetic tests cover layer and overprint state across windows, long operands, inline images, failed decoding, document limits and incomplete tails. Empty and populated metadata key presence is regression-tested, and CI checks the immutable backend source.
     Invisible text in a PDF (rendering mode 3, which carries a scanned page's recognized text over the scan) opens as type with no fill or stroke, a run of its own beside visible text on the same line; text that can't come in as type (opened as outlines, Type 3 fonts) is left out with a warning. PDF export, and printing, write characters with no fill or stroke as real text in rendering mode 3, so they draw nothing but stay selectable and searchable, also in PDF/X-1a and PDF/A-2b (#2675, `pdf/src/tests/import_fidelity.rs`, `pdf/src/tests/realtext.rs`).
     A PDF, or a `.ai` read through its PDF part, whose spot inks have the same values but different names opens with a spot swatch for each ink. The solid fills and strokes of paths and type, and the stops of gradients on paths, link to the swatch of the ink they are painted in, the points of a gradient mesh in one of these inks keep the ink's color, and PDF export writes each of these inks as its own separation (`pdf/src/tests/import_color.rs`). Paint in these inks inside pattern cells, soft masks and type 3 glyphs, and gradients on type in these inks, still import as RGB. Type in one of these inks can also import as RGB when type in another Separation or DeviceN ink is shown with no path drawn between them.
+    PDF export writes a linear or radial gradient that mixes a spot color with process colors, or two spot colors, as a DeviceN shading of the spot inks and the process inks its stops use, so each spot ink prints on its own plate; an off-center midpoint is written as a stop with the inks halfway between its two stops. A gradient that mixes a spot color with RGB or Lab colors is still written without the spot ink, with a warning (`engine/src/tests/linked_stops.rs`).
   - `.vectorcraft` (lossless JSON, compressed, atomic saves, save down to v1), Save As with a format chooser, Save a Copy, Revert, templates, Data Recovery after a crash (browser tabs hold their copies with Web Locks too, so a background tab's copies are never taken for a crash's, and a tab repairs copies another removed when it resumes: #367), background save and export. The desktop app writes its warnings and errors (graphics adapter, a lost graphics device, panics the guard recovered from) to standard error and to `logs/vectorcraft.log` next to the preferences, the two previous runs kept and each file capped at 16 MiB, so a launch without a terminal still leaves something to attach to a bug report (#465). Native documents with a lot of path data (traced or generated art, in layers, symbols or patterns) are saved as compact JSON, about a fifth of the size; small documents stay pretty-printed (#1492). Saving to a network drive on macOS falls back to a plain fsync when the share refuses a full flush (#1625, part of #1620; not confirmed on a real share), and a save still succeeds when a folder refuses the temporary file, the rename over the document or copying permissions: the content is written elsewhere first and copied in, and a failed copy keeps the temporary file and names it (#1649, part of #1620). A document opened by a relative path finds its relatively linked images, and `vectorcraft-cli convert` lists what reading the input left out as `importWarnings` (#1606, fixes #1604).
     A name typed in the desktop save panel without the file type's extension gets it, and when a file of that name exists, which the panel did not ask about, Save, Save As, Save a Copy, Save as Template, the exports and every other write through a save panel ask before replacing it (dialog `replaceFile`; Cancel writes nothing).
     A name returned by the desktop save panel with the extension of a format the save does not write has it replaced with the saved format's: Save As of a new document saves `card.ait` and `card.png` as `card.vectorcraft`, and Save as Template saves `card.pdf` as `card.vctemplate`. An extension that names no format stays part of the name (`notes.v2` is saved as `notes.v2.vectorcraft`).