Faux bold when the font does not support it

[christianrondeau]

Prerequisites

• I have written a descriptive pull-request title
• I have verified that there are no overlapping pull-requests open
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• I have provided test coverage for my change (where applicable)

Description

A follow-up to #542 - should be merged after the italic one.

Bold is much more complex than italic, because it requires dilating glyphs, although it does not require changing the advance (I thought it would require it but comparing to Chrome, it looks fine with just widening the glyphs).

Disclaimer: this code was written using AI (Copilot with Claude Opus 4.8), and reviewed by me. However, while the italic PR is fairly simple to understand, I don't have the knowledge to personally fully verify EmboldeningGlyphRenderer. This being said, the logic looks sound and renders correctly.

Thanks for your consideration!

[codecov]

Codecov Report

❌ Patch coverage is 96.69421% with 4 lines in your changes missing coverage. Please review.
✅ Project coverage is 84%. Comparing base (38b3e5a) to head (6e54360).
⚠️ Report is 3 commits behind head on main.

Files with missing lines	Patch %	Lines
...Labors.Fonts/Rendering/EmboldeningGlyphRenderer.cs	95%	0 Missing and 4 partials ⚠️

Additional details and impacted files

@@          Coverage Diff           @@
##            main    #543    +/-   ##
======================================
  Coverage     84%     84%            
======================================
  Files        332     333     +1     
  Lines      24708   24815   +107     
  Branches    3618    3640    +22     
======================================
+ Hits       20813   20917   +104     
  Misses      3009    3009            
- Partials     886     889     +3     

Flag	Coverage Δ
unittests	84% <96%> (+<1%)	⬆️

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[JimBobSquarePants]

Thanks for this. I’m going to accept this PR as an initial integration point, but with some important caveats.

Synthetic bold should be owned by Fonts so that all renderers receive the same glyph geometry. However, the current emboldening implementation should not be treated as final. I want to follow browser layout behavior, meaning synthetic bold should not change layout advance, but I also want the actual outline dilation logic to be much closer to FreeType’s outline emboldening algorithm.

So I’m going to merge this to unblock the feature, then follow up by replacing/refining the dilation algorithm and adding stronger tests around counters, mixed contour winding, CFF/TrueType outlines, and bounds behavior.

[christianrondeau]

Thank you; let me know if you'd like me to spend more time on any of it!