Implement CGPath related C API

[Kyle-Ye]

No description provided.

[augmentcode]

🤖 Augment PR Summary

Summary: This PR implements CoreGraphics-path-related C APIs in OpenSwiftUI_SPI and wires them into Path string parsing.

Changes:

• Replaces the previous stubbed __CGPathParseString with a real implementation and exposes it (renamed) as _CGPathParseString.
• Adds _CGPathCopyDescription to serialize a CGPath into the project’s command-string format (with optional coordinate rounding).
• Adds _CGPathCreateRoundedRect to construct rounded-rect paths with radius clamping behavior.
• Updates Sources/OpenSwiftUICore/Shape/Path.swift to call _CGPathParseString for LosslessStringConvertible parsing.
• Updates Package.swift so OPENRENDERBOX_RENDERBOX is defined consistently for Swift, C, and C++ compilation settings.
• Removes the old __CGPathParseString header/implementation files.
• Adds a dedicated SPI test suite covering parsing, description generation, and rounded-rect edge cases.

Technical Notes: The new API lives under the OpenSwiftUI_SPI CoreGraphics overlay (modulemap umbrella) and is guarded for Darwin/CoreGraphics availability.

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[augmentcode]

Review completed. 3 suggestions posted.

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[codecov]

Codecov Report

❌ Patch coverage is 0% with 1 line in your changes missing coverage. Please review.
✅ Project coverage is 28.39%. Comparing base (7b4928d) to head (5c9ff28).
⚠️ Report is 3 commits behind head on main.

Files with missing lines	Patch %	Lines
Sources/OpenSwiftUICore/Shape/Path.swift	0.00%	1 Missing ⚠️

Additional details and impacted files

@@             Coverage Diff             @@
##             main     #676       +/-   ##
===========================================
+ Coverage   13.81%   28.39%   +14.57%     
===========================================
  Files         559      584       +25     
  Lines       32555    35186     +2631     
===========================================
+ Hits         4499     9992     +5493     
+ Misses      28056    25194     -2862     

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