Update Font modifiers

[Kyle-Ye]

Summary

• implement font descriptor and resolved-trait handling across the remaining Font modifiers
• add non-CoreText descriptor shims and platform-unavailable fallback warnings
• use semantic CoreText trait and variation-axis representations for leading, weight, width, and variable-font settings

Impact

This completes the modifier paths used to derive CoreText font descriptors while keeping unsupported platforms on an explicit, non-crashing fallback path.

Validation

• parsed the updated Swift sources with the package availability macros
• ran git diff --check

[augmentcode]

🤖 Augment PR Summary

Summary: This PR completes the remaining Font modifier implementations needed to derive CoreText font descriptors, while providing explicit fallback behavior on platforms without CoreText.

Changes:

• Implements descriptor and resolved-trait handling for many Font modifiers (italic/bold/leading/weight/width/grade/variation/features).
• Adds additional private CoreText hooks and constants used for trait extraction and UI font descriptor creation.
• Introduces non-Darwin shims for CTFont/CTFontDescriptor and moves placeholder types out of the CoreText-specific file.
• Uses CoreText trait and variation-axis representations for leading/weight/width and variable-font settings.
• Adds explicit platform-unavailable fallback warnings instead of crashing on unsupported platforms.

Technical Notes: The CoreText path now builds descriptors via symbolic traits, feature/variation attributes, and UI-type descriptor creation; non-CoreText builds retain API surface via minimal placeholder types and warnings.

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[augmentcode]

Review completed. 3 suggestions posted.

Comment augment review to trigger a new review at any time.

[codecov]

Codecov Report

❌ Patch coverage is 1.54639% with 191 lines in your changes missing coverage. Please review.
✅ Project coverage is 26.29%. Comparing base (deaf7fa) to head (b6c78b6).

Files with missing lines	Patch %	Lines
.../OpenSwiftUICore/View/Text/Font/ModifiedFont.swift	1.57%	187 Missing ⚠️
...wiftUICore/Shim/CoreFoundationNonDarwinShims.swift	0.00%	4 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #943      +/-   ##
==========================================
- Coverage   26.35%   26.29%   -0.07%     
==========================================
  Files         723      724       +1     
  Lines       51232    51368     +136     
==========================================
+ Hits        13504    13505       +1     
- Misses      37728    37863     +135     

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