Add NSAttributedString text measurement and drawing implementation

[Kyle-Ye]

Summary

Implements the NSAttributedString-based text measurement and drawing path used by Text, plus the supporting CoreText/UIFoundation SPI shims and a test suite covering the new behavior.

Changes

• Text+StringDrawingContext.swift (new): ResolvedStyledText.StringDrawing implementation, including the NSAttributedString measure path (maxFontMetrics, Metrics, layout margin and pixel-length rounding).
• Text+Spacing.swift (new): Spacing.textSpacing(maxFontMetrics:idealMetrics:layoutProperties:), covering standard / uniform line height sizing and horizontal / vertical writing modes.
• CoreFont.swift (new): CoreText font helpers used by the measurement path.
• Text+Measure.swift: removed, superseded by the string-drawing context.
• SPI shims: added CTFontSPI.h and NSStringDrawing_Private.h declarations required by the above.

Tests

NSAttributedStringTests (11 tests across 4 suites):

• MaxFontMetricsTests — font run aggregation, missing font attributes, and clipping outsets gated on Semantics.TextRenderingMetrics.
• MetricsTests — single line, wrapped lines, line limit, layout margins, and baseline rounding to the pixel length.
• TextSpacingTests — standard sizing under both Semantics.TextSpacingUIKit0059v2 states, uniform line height half-leading split, and vertical writing mode edge mapping.

Assertions use absolute reference values measured from Helvetica 16 / 24 rather than re-deriving the implementation's math, and all CGFloat / Double comparisons go through Numerics.isApproximatelyEqual.

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🤖 Augment PR Summary

Summary: This PR implements the NSAttributedString-backed text measurement + drawing path used by Text, including spacing computation and platform shims needed for CoreText/UIFoundation internals.

Changes:

• Added Text+StringDrawingContext.swift with a full ResolvedStyledText.StringDrawing implementation (measurement cache, baseline/pixel rounding, layout margins, drawing via NSStringDrawingContext).
• Added Text+Spacing.swift to compute text-to-text / baseline / edge spacing for horizontal and vertical writing modes, including uniform line-height behavior.
• Added CoreFont.swift with CoreText font helpers used by the measurement path (including limited-line height calculation).
• Removed the older Text+Measure.swift WIP implementation in favor of the new string-drawing context.
• Extended SPI shims (CTFontSPI.h, NSStringDrawing_Private.h) to access required private CoreText/UIFoundation properties.
• Updated Text+View.swift to construct styled text via ResolvedStyledText.styledText(...).
• Expanded NSAttributedStringTests with suites covering max font metrics, metrics rounding/margins, and text spacing semantics across feature flags.

Technical Notes: Measurement/drawing relies on CoreText/UIFoundation availability on Darwin and uses cached layouts + pixel-length rounding for stable baseline/height behavior.

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Review completed. 2 suggestions posted.

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[augmentcode]

kitFont() calls attribute(_:at:0,...) unconditionally, which will trap for empty attributed strings (length == 0). Consider guarding on length > 0 (or otherwise handling empty strings) since kitFont()/limitedFontHeight(by:) are package and could be called on empty storage.

Severity: medium

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[augmentcode]

When widthIsFlexible is true, width is set to requestedSize.width even if it’s .infinity (e.g. sizeThatFits uses .infinity for unspecified dimensions), which can leak an infinite width into Metrics.size. Consider only forcing the width when the requested width is finite, so flexible justification doesn’t produce an invalid/infinite size in unconstrained measurement.

Severity: medium

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Review completed with 1 suggestions.

[augmentcode]

MetricsCache reuses entries solely by requested size, but each cached Metrics already has the caller's layoutMargins baked into size and baselines. A later metrics(in:layoutMargins:) call with different margins can hit this entry and return dimensions or line counts for the wrong content rectangle, especially because the measurement width subtracts the margins before layout.

Severity: medium

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[codecov]

Codecov Report

❌ Patch coverage is 41.97002% with 271 lines in your changes missing coverage. Please review.
✅ Project coverage is 26.50%. Comparing base (cc9f9b0) to head (c43b9f0).
⚠️ Report is 1 commits behind head on main.

Files with missing lines	Patch %	Lines
...ore/View/Text/Text/Text+StringDrawingContext.swift	36.08%	248 Missing ⚠️
...rces/OpenSwiftUICore/View/Text/Font/CoreFont.swift	0.00%	22 Missing ⚠️
...ces/OpenSwiftUICore/View/Text/Text/Text+View.swift	0.00%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #947      +/-   ##
==========================================
+ Coverage   26.27%   26.50%   +0.23%     
==========================================
  Files         726      728       +2     
  Lines       51669    52108     +439     
==========================================
+ Hits        13575    13812     +237     
- Misses      38094    38296     +202     

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