feat: typography controls — lineHeight, fontThicken

[griffinwork40]

Typography controls: lineHeight + fontThicken

Adds two user-configurable typography fields and extracts the font-sizing
methods into a dedicated extension file.

What ships

fontThicken (bool, default false)

Applies CGContextSetFontSmoothingStyle(48) — medium sub-pixel dilation —
before SwiftTerm's glyph draw pass. This is the same style Terminal.app uses
(and iTerm2, MacVim, Emacs). It compensates for the AA bleed direction on
white-on-dark rendering that makes terminal text appear thinner than the same
font on a light background.

• Call is confined to UmberTerminalView.draw(_:) so the CGContext is
  always the one live for the current draw call.
• Guarded by the user preference: when false, the override is a zero-cost
  pass-through to super.draw(dirtyRect).
• Private API, same as Terminal.app/iTerm2/Emacs — linker surfaces a
  missing-symbol error at build time if Apple removes it (no silent runtime trap).
• Vendor note: MacTerminalView.swift:954 already declares open override func draw,
  so no vendor patch is needed.

lineHeight (CGFloat 0.8–2.0, default 1.0)

Multiplier on the font's natural line metrics, wired to SwiftTerm's public
lineSpacing property. 1.2 = 20% extra leading. The setter internally
calls resetFont() → computeFontDimensions() → processSizeChange, which
correctly resizes the terminal grid — no partial relayout or clipped rows.

applyTypography(_:) is called from apply(config:) after setFontSize
because lineSpacing needs the font already installed on the view.

Extraction: TerminalPane+Typography.swift (new file)

TerminalPane.swift was at 346/350 LOC. The font-sizing block (constants,
setFontSize, adjustFontSize, resetFontSize, currentFontSize) moved to
TerminalPane+Typography.swift alongside the new applyTypography(_:).

Access fix: config and fontSize were private (file-scoped in Swift).
Changed to internal (default, no keyword) so the cross-file extension can
reach them. A comment in the source explains why — Swift private is
file-scoped, not class-scoped, unlike many other languages.

What does NOT ship (deferred)

• Ligature control: SwiftTerm's attribute cache (attributes, urlAttributes)
  is a private var on TerminalView with no public setter. Disabling ligatures
  would require a vendored patch to getAttributes(_:withUrl:). Omitted rather
  than half-implemented; documented in TerminalPane+Typography.swift header.

File sizes

All files ≤ 350 LOC. TerminalPane.swift went from 346 → 317 lines.

Checklist

• swift build passes (zero errors, pre-existing warnings only)
• bash Scripts/check-file-size.sh — all 117 files within ceiling
• fontThicken and lineHeight documented in StarterConfig.swift with usage notes
• All @MainActor, no try!
• Vendor draw(_:) is already open — no patch required