Integrate Babylon React Native into the Babylon Native monorepo

[CedricGuillemet]

Summary

Bring Babylon React Native into the Babylon Native monorepo at Embedding/ReactNative, preserving the Playground name and BRN's rewritten Git history.

• Import BabylonJS/BabylonReactNative master at 721b33ff6303fad3757a9c645abd8f13e2a3fcba, excluding .github and .vscode. Keep its README and original license/security information; reuse the root contribution policy. No BRN tags are pushed.
• Use the current checkout for development. The shared CMake wrapper finds the enclosing checkout from source and npm-installed local modules; npm consumers retain the disconnected Repo/deps layout and existing binary library paths. Android no longer forces disconnected mode for developer builds; packaged builds still enable it in the shared wrapper.
• Replace the pinned Babylon Native ZIP download with a curated source copy into Package/Assembled, excluding apps, documentation, install scripts, CI, and Embedding/ReactNative itself. Preserve CMake dependency staging/pruning and remove ZIP/rename dependencies.
• Port Android, iOS, Windows, assembly, packaging, and package-consumer GitHub Actions. All 11 workflows are flat .github/workflows/reactnative-*.yml files with React Native display names; GitHub does not discover workflows in subdirectories. Update reusable-workflow references, documentation links, and the PR path filter accordingly.
• Extend Azure nightly with BabylonNative and ReactNative stages, preserving existing native jobs and mirroring the reactnative-*.yml assembly, tarball, platform-build, and package-consumer workflows. Merge current upstream changes while preserving its Hermes nightly jobs and native publication gating inside the BabylonNative stage.
• Publish releases tagged react-native/vX.Y.Z as npm version X.Y.Z through reactnative-publish.yml; provide manual preview publishing through reactnative-publish-preview.yml.
• Exclude generated Build, nested _deps, and packaged native source/dependency trees from module TypeScript compilation.

Temporary CI wiring -- remove before merge

.github/workflows/ci.yml unconditionally calls ./.github/workflows/reactnative-pr.yml for this draft. This is marked # TEMPORARY: remove before merge and included in the aggregate check. Remove the temporary wiring before merging. The separate path-filtered entry avoids duplicate builds while this call exists and automatically resumes direct execution when it is removed.

Validation and CI fixes

• Full npx gulp buildAssembled and validateAssembled passed. npm pack passed; verified required native/dependency/license files and absence of recursive React Native copies or Git metadata.
• Configured the assembled wrapper offline and built Graphics in Release. Confirmed the packaged iOS entry selects its bundled toolchain without downloads.
• Configured source and npm-installed developer wrappers and built Graphics. Root native CMake files are unchanged and do not include React Native.
• Playground Jest suite: 3 tests passed. Nested Playground npm-root resolution was checked. No regression test was added, as requested.
• Actionlint, all 11 flat workflow names, reusable-workflow references, job IDs, path filter, Azure YAML, and whitespace checks passed after the rename. After merging upstream, verified that the BabylonNative stage's jobs exactly match upstream and the ReactNative stage is unchanged.
• gulp setupDev passed with physically populated iOS Build, Android .cxx/_deps, and packaged Repo/deps trees; their TypeScript files were excluded. Temporary verification files were removed.
• All React Native jobs passed on 7efcb661 in CI run 38886973160: Android, iOS, and Windows source builds; assembly; packaging; and all three packaged-consumer builds. This confirms both the generated-TypeScript exclusions and the Android developer FetchContent fix.
• Local Android Gradle configuration accepts the follow-up and reaches normal dependency fetching; a full local Android configure is blocked by the SDK Ninja Windows 260-character path limit. Packaged offline configuration was rechecked successfully; hosted Android source and packaged-consumer builds passed.
• Azure nightly cannot be executed from this environment.

Repository setup and follow-ups

Publishing requires a BabylonNative repository secret named NPM_TOKEN with permission to publish @babylonjs/react-native.

Not included: npm trusted publishing, the future of the old BabylonReactNative repository, or switching BRN to consume Babylon::Embedding.