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     

☔ View full report in Codecov by Sentry.
📢 Have feedback on the report? Share it here.

🚀 New features to boost your workflow:

• ❄️ Test Analytics: Detect flaky tests, report on failures, and find test suite problems.