Use CoreText run font fallback for glyph paths

[Weixi779]

Closes #109

[codecov]

Codecov Report

❌ Patch coverage is 80.00000% with 2 lines in your changes missing coverage. Please review.
✅ Project coverage is 88.47%. Comparing base (17d55c1) to head (14535c5).
⚠️ Report is 3 commits behind head on main.

Files with missing lines	Patch %	Lines
SwiftDraw/Sources/Utilities/CGPath+Segment.swift	80.00%	2 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #110      +/-   ##
==========================================
- Coverage   88.48%   88.47%   -0.01%     
==========================================
  Files         148      148              
  Lines        9966     9975       +9     
==========================================
+ Hits         8818     8825       +7     
- Misses       1148     1150       +2     

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[Weixi779]

Hi @swhitty,
Codecov reports low patch coverage because the new run-level font fallback path isn’t exercised by existing tests. I can add
a small CoreText-based test to assert that CTRun fonts are used (e.g. a CJK glyph renders via fallback) if you’re okay with
adding a platform-specific test gated by canImport(CoreText). Please let me know your preference.

[swhitty]

That's fine, I am able to merge anyway, but do you have a sample SVG I can add to the samples bundle that demonstrates this issue & fix?

[Weixi779]

Thanks @swhitty! So quickly reply.
I’ve added a sample to Samples.bundle: cjk-fallback.svg.

It uses mixed CJK text with font-family="Helvetica" (often missing CJK glyphs) to trigger CoreText fallback:

• Chinese: 敏捷的棕色狐狸跳过懒狗，窗前明月光照桌上旧书
• Japanese: いろはにほへとちりぬるを　わかよたれそ　つねならむ
• Korean: 다람쥐 헌 쳇바퀴에 타고파, 한글도 잘 보이나요?

The CJK lines may render missing glyphs; after this change, CoreText’s run-level fallback renders them correctly.

[swhitty]

Thank you 🙏