Add CGVector type

[Kyle-Ye]

Summary

• Add CGVector struct with dx/dy properties matching Darwin's CoreGraphics API
• Conform to Equatable, Sendable, Hashable, and CustomDebugStringConvertible

Test plan

• Verify build succeeds locally
• CI passes on all platforms (macOS, Linux)

[codecov-commenter]

Codecov Report

❌ Patch coverage is 0% with 11 lines in your changes missing coverage. Please review.
✅ Project coverage is 70.11%. Comparing base (a36ba82) to head (c284db3).

Files with missing lines	Patch %	Lines
Sources/OpenCoreGraphics/CGVector.swift	0.00%	11 Missing ⚠️

Additional details and impacted files

@@ Coverage Diff @@## main #7 +/- ##
==========================================
- Coverage 71.35% 70.11% -1.25% 
==========================================
Files 13 14 +1 Lines 618 629 +11 ==========================================
Hits 441 441 - Misses 177 188 +11 

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