Fix CoreGraphics imports for modern Xcode

[XStylus]

Summary

This updates two OmniFoundation headers to import CoreGraphics/CGGeometry.h directly before using CoreGraphics geometry types.

OFExtent.h and OFGeometry.h use types such as CGFloat, CGRect, and CGSize. On modern Xcode/macOS SDK combinations, those types are not reliably available through the existing macOS-side Foundation/NSGeometry.h import alone.

Making the CoreGraphics import explicit avoids relying on an indirect SDK include relationship.

Changes

• Imports CoreGraphics/CGGeometry.h once at the top of OFExtent.h.
• Imports CoreGraphics/CGGeometry.h once at the top of OFGeometry.h.
• Leaves the existing platform-specific Foundation/NSGeometry.h and UIKit/UIGeometry.h imports in place.

Verification

Verified locally with Xcode 26.5:

xcodebuild -version
# Xcode 26.5
# Build version 17F42