Part of #394: Appearance of Black › On Screen shows pure CMYK black as RGB black

Preferences › Appearance of Black › On Screen was stored and never read.
Display All Blacks Accurately (the default) shows pure CMYK black, black
ink alone at 100%, through the working CMYK profile, a dark grey, as the
canvas always did. Display All Blacks as Rich Black now shows it on the
canvas as RGB black: fills, strokes, type, gradient stops, effect colours
and pattern tiles, and in CMYK documents drawn by ink planes a pixel
whose inks come out as black ink alone. Rich blacks, black tints and
placed images show the same either way; exports and printing are
unchanged (Printing / Exporting is a separate option).

The renderer gets RenderOptions::rich_black, painted with a new
Ink::RichBlack (screen colours with pure black as RGB black) and applied
by ink::compose. The canvas sets it from blackOnScreen and keeps it in
its cache key, so changing the preference redraws.

A headless frame test reads the canvas raster: K100 shows its profile
grey by default, RGB black with rich, a rich black unchanged, and grey
again with accurate. A render test covers the opaque and ink-plane paths
and checks that rich blacks, tints and images don't change.

Docs: docs/mcp.md (blackOnScreen), docs/target-app-parity.md,
docs/gaps.md, docs/ui-parity.md.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

crates/render/src/pattern.rs

@@ -38,7 +38,7 @@ struct TileEntry {
 
 struct PatternCache {
     /// Per ink ([`Ink`] as index), each within [`lru::PATTERN_BUDGET`], least recently used first out.
-    tiles: [FrameCache<String, TileEntry>; 3],
+    tiles: [FrameCache<String, TileEntry>; 4],
     renderer: Option<Box<Renderer>>,
     depth: u32,
 }