fix(macos): include React-RCTUIKit.podspec in package files array

[tvinhas]

Summary

packages/react-native/package.json lists three macOS-fork podspecs in the files array — React-Core.podspec, React-Core-prebuilt.podspec, React.podspec — but omits React-RCTUIKit.podspec, which was introduced by #2847
(RCTUIKit module-ification, merged March 2026). As a result the file lives in the source tree but is excluded from the npm publish.

Any downstream consumer of a published `react-native-macos@0.83` hits this immediately:

```
[!] No podspec found for `React-RCTUIKit` in `../node_modules/react-native-macos/`
```

`pod install` fails before reaching any actual install step.

Discovered while running react-native-test-app's `example-macos` against the fork's `0.83-merge` via a local Verdaccio registry — the file shows up in the source tree
(`packages/react-native/React-RCTUIKit.podspec`) but never lands in `node_modules/react-native-macos/` after install.

Fix is a one-line addition to the `files` array, right next to the other root-level podspecs. Closes the consumer-install gap for any downstream user (RNTA, Epistles, anyone else) once 0.83 ships to npm.

Test Plan

• Before: `npm pack` on `packages/react-native` produces a tarball without `React-RCTUIKit.podspec`. RNTA's `example-macos` `pod install` fails with "No podspec found for React-RCTUIKit".
• After: `npm pack` includes the file; `pod install` resolves the React-RCTUIKit pod from `../node_modules/react-native-macos/React-RCTUIKit.podspec` and continues. Validated locally with RNTA's `example-macos` building
  cleanly through to a `ReactTestApp.app` artifact.

Related

• refactor: move RCTUIKit into a separate module #2847 — introduced `React-RCTUIKit.podspec` (and the corresponding `React-RCTUIKit` pod dep in `React-Core.podspec`), but didn't update the `files` array.
• fix(macos): expose React-RCTUIKit module to RCTSwiftUI Swift consumers #2958 — wires `React-RCTUIKit` into `RCTSwiftUI.podspec`; depends on this podspec being available to consumers.
• Road to 0.83 #2901 — Road to 0.83 tracking issue (unblocks downstream consumers in general).