Initial commit of Cxx API snapshots (#56152)

Summary:
Pull Request resolved: #56152

## Summary

Introduces initial snapshots of the React Native C++ public API. These snapshots capture the public-facing C++ API surface as `.api` files, enabling tracking of API changes across diffs.

Six snapshot files are generated, covering three targets in two build variants each:

| Target | Debug | Release |
|---|---|---|
| **ReactCommon** (shared) | `ReactCommonDebugCxx.api` | `ReactCommonReleaseCxx.api` |
| **ReactAndroid** | `ReactAndroidDebugCxx.api` | `ReactAndroidReleaseCxx.api` |
| **ReactApple** | `ReactAppleDebugCxx.api` | `ReactAppleReleaseCxx.api` |

Debug and Release variants differ in preprocessor definitions (e.g., `DEBUG`/`REACT_NATIVE_DEBUG` vs `NDEBUG`/`REACT_NATIVE_PRODUCTION`), which can affect conditional compilation and the resulting public API surface.

The snapshots are generated by the `scripts/cxx-api` tool, which runs Doxygen on the C++ headers and codegen output, then produces a normalized text representation of all public types, functions, enums, and class hierarchies.

Changelog: [Internal]

Reviewed By: cipolleschi

Differential Revision: D97296573

Author: coado