fix(macos): include React-RCTUIKit.podspec in package files array (#2963)

## 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](https://github.com/microsoft/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

- #2847 — introduced \`React-RCTUIKit.podspec\` (and the corresponding
\`React-RCTUIKit\` pod dep in \`React-Core.podspec\`), but didn't update
the \`files\` array.
- #2958 — wires \`React-RCTUIKit\` into \`RCTSwiftUI.podspec\`; depends
on this podspec being available to consumers.
- #2901 — Road to 0.83 tracking issue (unblocks downstream consumers in
general).