Use CoreText run-level font fallback for glyph paths

[Weixi779]

Problem Background

I’m an Asian developer and ran into a common headache: CJK text sometimes showed missing glyphs when the requested font didn’t
have them. Looking at the code, we weren’t using CoreText’s per-run fallback, so we relied on brittle, hard-coded font names
that vary by platform/locale.

Repair plan

• In CGPath+Segment.swift, use the CTFont attached to each CTRun (kCTFontAttributeName) when drawing glyphs; if it’s
  missing, fall back to the requested font.
• No API changes, just better resilience for mixed scripts and missing-glyph cases across platforms.

Testing

• Local build succeeds (e.g. SWIFTPM_PACKAGE_CACHE=/tmp/swiftpm-cache swift build --scratch-path /tmp/swiftpm-scratch).
• Manual CLI renders with CJK SVG content display correctly.