fix(color): gradients between process colours blend as inks, as InDesign shows them

LYH Studio episode 6 builds every gradient from C=0 M=100 Y=0 K=0 to
C=100 M=0 Y=0 K=0. InDesign blends a gradient's process stops as inks:
halfway is C 50 M 50, a light lilac. DesignCraft converted both ends to
RGB and blended there, so the middle came out a darker violet (117, 88,
189 instead of 135, 119, 181), and every Episode 6 gradient looked
different. A midpoint between CMYK stops also became an RGB colour, so
the PDF shading turned RGB with a conversion warning.

- color: `Color::lerp` blends CMYK with CMYK (or grey), and grey with
  grey, in ink space; anything with RGB still blends in RGB.
- `Gradient::display_stops`: the expanded stops plus samples through
  each span that isn't RGB at both ends, for renderers that interpolate
  in RGB; the screen renderer uses it.
- pdf: a midpoint's expanded stop now keeps the stops' space; the
  colour-space test expects no conversion for CMYK or grey midpoints.

Co-Authored-By: Claude Opus 5.5 <[email protected]>

docs/target-app-parity.md

@@ -436,7 +436,7 @@ It is evidence of breadth: each item exists. It is not a claim of InDesign-level
 
 Task notes that headed ROADMAP.md:
 
-- FID1: PDF export normalizes mixed emitted gradient color spaces to RGB with an explicit warning; homogeneous device-space gradients retain their components. Linear/radial, midpoint, archival and print policies have synthetic regression coverage.
+- FID1: PDF export normalizes mixed emitted gradient color spaces to RGB with an explicit warning; homogeneous device-space gradients retain their components. Process stops (CMYK, grey) blend as inks on screen and at midpoints, so a midpoint keeps the shading CMYK. Linear/radial, midpoint, archival and print policies have synthetic regression coverage.
 - AR1.1: Kashida justification tests verify space fallback without Tatweel fonts using an isolated font database, while retaining elongation assertions when fonts support it.
 - AR1: Arabic IDML controls now retain character direction, kashida switches, diacritic offsets/forms, paragraph policy values and independent story/table directions. Shaping preserves joining context and script/language selection; paragraph-level bidi, contextual digit conversion, mark-safe elongation, RTL columns/tables and physical column insertion have regression coverage. Vendor-specific justification and diacritic presets remain explicit preflight warnings; see docs/arabic-typography.md.
 - CJK1: IDML composite fonts now retain character mappings and metrics; explicit aki, tsume, jidori, em alignment/leading, named/custom kinsoku boundaries, hanging punctuation, tate-chu-yoko offsets and kenten symbols flow through composition and IDML export. Automatic kerning no longer imports inactive numeric values as enormous manual spacing. Composite definitions are editable through commands with undo/cache invalidation. Mojikumi definitions are retained and unsupported spacing-table / push-in-push-out policies are reported by preflight. Full InDesign CJK composition parity, frame-grid layout and detailed ruby remain unfinished. Warichu sets the selected run in smaller lines inside the line after the line is broken.