Custom heading typeface and heading text color

[dylandeheer]

Motivation

An embedder that sets serif display headings over a sans body can't get there today: the theme routes every color through MarkdownEditorTheme except heading text (headings inherit bodyText from the view-level base attributes), and HeadingStyle covers every heading metric (per-level multipliers, top spacing) except the typeface — headings always derive from the single base fontName with the bold trait added. Those are the only two gaps we hit embedding the engine; this PR closes both without changing any default.

What's added

Two optional knobs, both nil by default:

• HeadingStyle.fontName: String? — PostScript face for heading text, resolved at the per-level multiplied size. The name is honored exactly, so the chosen weight survives (pick a -Bold / -Semibold face for heavier headings); an unresolvable name falls back to the stock bold base font, mirroring TaskCheckboxStyle's symbol fallback. Emphasis inside a heading still composes on top of the configured face — bold/italic add traits while the family and per-level size are kept (# **n*o*des** stays consistent).
• MarkdownEditorTheme.headingText: NSColor? — foreground for heading text only. The # glyphs stay on the existing headingMarker, and inline constructs inside a heading (links, inline code, extension spans) keep their own ink — the styler paints the heading range before the marker loop and the inline descent, so the existing later-range-wins layering does the rest.

Behavior preservation

With both knobs nil the styler emits value-identical [StyledRange] output to 0.11.0 — same font resolution path, same emission order, no extra ranges. Covered by a test comparing explicit-nil configs against .default attribute-by-attribute.

Tests

Tests/MarkdownEngineTests/HeadingFontAndColorTests.swift (8 tests, in the existing AST-styler test style):

• custom face applied verbatim at the multiplied size per level; body text untouched
• emphasis inside a custom-face heading keeps family/size and adds traits
• unresolvable face falls back to the stock heading font
• headingText colors content only; # markers keep headingMarker; body keeps inheriting bodyText; links keep link ink; emphasis keeps the heading ink
• nil knobs: stock font, no foreground range, value-identical output to .default

swift build and swift test are green locally (the four ScrollingHeaderController animation-timing expectations fail identically on the pristine 0.11.0 tag in my sandbox, so they're environmental, not from this change).

Notes

• Branched from the 0.11.0 tag so we can pin the exact revision while this is reviewed — happy to rebase onto main (only the CHANGELOG insertion point should touch).
• Opened as a draft in the spirit of CONTRIBUTING's "issue first for non-trivial features" — treating this as the design conversation. Happy to adjust API names/placement or wire a Demo toggle if you'd like one.
• CHANGELOG entry added under [Unreleased]; DocC comments on both new public fields.

Made with Cursor

Follow-ups

This PR turned out to be the first of a family: embedding the engine against a full design spec surfaced a few more places where a knob (or a bugfix) was needed. Each is opened as its own focused draft, stacked on this branch (their diffs over this PR are single-feature; every default preserves today's rendering exactly):

• Fix: theme link color never reaches the screen #122 — fix: the theme's link color never reaches the screen (AppKit's linkTextAttributes default overrides it)
• Opt-in list indent grid (ListStyle.markerTextGap) #123 — opt-in list indent grid (ListStyle.markerTextGap)
• Ordered-marker lettering per depth and list marker ink #124 — ordered-marker lettering per depth + theme.listMarker ink
• Checked-task treatment knobs (strikethrough gate, label ink, checkbox tints) #125 — checked-task treatment (strikethrough gate, label ink, checkbox tints)
• Blockquote bar, indent, and content color knobs #126 — blockquote bar/indent/content-color knobs
• Heading line-height and below-heading spacing knobs #127 — heading line-height + below-heading spacing knobs
• Code font and background knobs #128 — code font + background knobs
• Table header background and rule theming slots #129 — table header background + rule theming slots