fix(0.83, rctuikit): make SwiftUI platform aliases visible to Swift (#3012)

## Summary

Follow-up fix to #3006, which merged before we had CI testing (oops).
Turns out `@compatibility_alias` is not compatible with Swift, so we
can't use it. Furthermore, `RCTUIKit` can't have Swift sources as SPM
doesn't support mixed language modules yet. So for now, let's move the
Swift-only usecase out to RCTSwiftUI, the caller.

## Test Plan

Validated locally with Xcode 26.4.1:

- Clean `RCTSwiftUI-macOS` Pods target build → **BUILD SUCCEEDED**.
- Full `RNTester-macOS` app build → **BUILD SUCCEEDED**.
- SwiftPM prebuild (`node scripts/ios-prebuild.js -s` then `-b -p
macos`) now compiles `React_RCTUIKit`, `RCTSwiftUI` and
`RCTSwiftUIWrapper` with **no "no such module"** error (the previous
failure point).

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>