[CPF] Use pure-Swift CG geometry on WASI/Linux/Android

[gn-adin-b]

What is the goal?

Stop importing full Foundation for CG geometry types on WASI/Linux/Android — that import reintroduces the ICU/Foundation payload the FoundationEssentials migration is trying to remove from the Wasm link closure.

How is it being implemented?

• Revert the CGFloat/CGPoint/CGSize/CGRect aliasing to Foundation's types on Linux/WASI/Android; use the existing pure-Swift structs instead (the same fallback already used on other platforms without CoreGraphics).
• Use canImport(FoundationEssentials) uniformly for the import instead of special-casing Linux/WASI/Android to full Foundation.
• Remove testGeometryTypesMatchFoundation, which asserted bit-for-bit equivalence with Foundation's geometry types and no longer holds now that these are distinct local types.

Testing

• swift build (native macOS)
• swift test --filter PolyfillTests (native macOS)