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>

docs/gaps.md

@@ -125,8 +125,8 @@ from G14 and the budget run of G15: **~170–270 h** one agent, ~50–80 h wall
 
 ## G12 Preferences and panels that don't act yet
 
-- **Missing:** ~17 Preferences options stored but not acted on (#394: Highlight Alternate Glyphs, Open
-  Documents As Tabs, Real-time Drawing, hyphenation language, Appearance of Black, devices…), Properties panel per
+- **Missing:** ~16 Preferences options stored but not acted on (#394: Highlight Alternate Glyphs, Open
+  Documents As Tabs, Real-time Drawing, hyphenation language, Appearance of Black for output, devices…), Properties panel per
   context, context menus beyond the canvas, Layers panel row menus and Collect for Export, SVG Interactivity.
 - **Impact:** settings that silently do nothing erode trust.
 - **Estimate:** 13–20 h.

docs/mcp.md

@@ -1615,7 +1615,10 @@ control channel's `ui.drag` scrubs), `showHomeScreen` (on by default: the Home s
 off, an empty window, and the Home button or `app.home` still shows the screen), `autoCollapseIconPanels` (off by
 default: on, a click away from a panel popped out of the icon column puts it away) and `displayPrintSize` (off by
 default: View › Actual Size / `view.actualSize` is one document point per screen point; on, one document inch fills
-96 screen points, an inch at the system's reference density whatever the display scaling).
+96 screen points, an inch at the system's reference density whatever the display scaling). `blackOnScreen`
+(Appearance of Black › On Screen, `accurate` by default) sets how the canvas shows pure CMYK black, black ink alone
+at 100%: `accurate` shows it through the CMYK profile, a dark grey, and `rich` shows it as RGB black. Rich blacks,
+black tints and images show the same either way, and exports are unchanged.
 
 `gridlineEvery` (72 pt by default) and `gridSubdivisions` (8 by default) set the document grid that View › Show Grid
 draws, and View › Snap to Grid snaps to its subdivisions. Changing either sets the grid of every open document.

docs/ui-parity.md

@@ -167,7 +167,7 @@ Layers panel.
 | SVG Interactivity, Retype, Comments, Version History panels | missing | the last two are cloud features (out of scope) |
 | Workspaces (save, reset, switch) | done | |
 | macOS save sheet: choose the format from a menu | done | #866; `apps/vectorcraft/src/mac_save_format.rs`; current suffix selects the initial format, changing it updates the filename; native SVG save and cancel after changing to PDF checked |
-| Preferences: every option acts | partial | ~17 stored but unused (#394) |
+| Preferences: every option acts | partial | ~16 stored but unused (#394) |
 
 ## Look and feel