Add documentation for Cxx API snapshot generation (#56146)

Summary:
Pull Request resolved: #56146

Adds documentation for the cxx-api snapshot generation pipeline, covering usage commands for generating and checking snapshots, and configuration.

It answers:
1. What the script does.
2. What the output looks like.
3. How to use it (generate, check).
4. How it works (Doxygen → parser → .api files).
5. How it can be configured (config.yml).
6. What to do when snapshots differ.

Changelog:
[Internal]

Reviewed By: huntie

Differential Revision: D96316004

fbshipit-source-id: b527606b87efb2af72680b572135a5fc595ab85e

Author: coado

scripts/cxx-api/README.md

@@ -0,0 +1,99 @@
+# scripts/cxx-api
+
+Python build pipeline for React Native's C++ (and Objective-C) API snapshots.
+
+## Overview
+
+`scripts/cxx-api` generates human-readable snapshots of React Native's public C++ API surface. It uses [Doxygen](https://www.doxygen.nl/) to parse C/C++/Objective-C headers and a custom Python parser to produce a simplified, sorted representation of every public symbol.
+
+The pipeline produces one `.api` snapshot file per configured **API view × variant** combination:
+
+| Snapshot | Description |
+|---|---|
+| `ReactCommonDebugCxx.api` | Platform-independent C++ API (debug) |
+| `ReactCommonReleaseCxx.api` | Platform-independent C++ API (release) |
+| `ReactAndroidDebugCxx.api` | Android-specific C++ API (debug) |
+| `ReactAndroidReleaseCxx.api` | Android-specific C++ API (release) |
+| `ReactAppleDebugCxx.api` | Apple-specific C++/Obj-C API (debug) |
+| `ReactAppleReleaseCxx.api` | Apple-specific C++/Obj-C API (release) |
+
+For each view, debug and release variants are generated with different preprocessor definitions (e.g. `REACT_NATIVE_DEBUG` vs `NDEBUG`), since `#ifdef` guards in the source headers can produce a different public API surface per variant.
+
+Snapshot files are committed to the repo under `scripts/cxx-api/api-snapshots/`.
+
+## Usage
+
+#### Generate snapshots
+
+Maintainers should run this command whenever making intentional C++ API changes:
+
+```sh
+python -m scripts.cxx-api.parser
+```
+
+#### Check snapshots against committed baseline
+
+This mode generates snapshots to a temporary directory and compares them against the committed `.api` files. It is designed for CI:
+
+```sh
+python -m scripts.cxx-api.parser --check
+```
+
+If any snapshot differs, a unified diff is printed and the process exits with a non-zero status. To fix a failing check, regenerate the snapshots with `python -m scripts.cxx-api.parser` and commit the updated `.api` files.
+
+## How it works
+
+The pipeline has two main stages:
+
+### 1. Doxygen XML generation
+
+Doxygen is configured via a generated config file (built from `.doxygen.config.template`) with the input directories, exclude patterns, and preprocessor definitions specified in `config.yml`. It outputs XML describing every symbol found in the headers.
+
+### 2. Snapshot parsing
+
+The Python parser (`parser/`) reads the Doxygen XML output and builds a scope tree of the public API surface. The tree is then serialized to a deterministically sorted, human-readable `.api` text format.
+
+## When to use it
+
+The snapshot should be regenerated whenever making intentional changes to the public C++ API surface. This includes additions, removals, and changes to files located in:
+- `xplat/js/react-native-github/`
+- `xplat/js/react-native-github/ReactCommon/`
+- `xplat/js/react-native-github/ReactAndroid/`
+- `xplat/js/react-native-github/ReactApple/`
+- `xplat/js/react-native-github/Libraries/`
+
+## Configuration
+
+All API views and their variants are defined in `config.yml`. Each view specifies:
+
+| Field | Description |
+|---|---|
+| `inputs` | Directories to scan for headers |
+| `exclude_patterns` | Glob patterns for files to skip |
+| `definitions` | Preprocessor macros to define |
+| `variants` | Named build variants (e.g. debug/release) with extra definitions |
+| `codegen` | Optional codegen platform (`android`, `ios`) to generate TurboModule/Component headers before scanning |
+| `private_directories` | Directories whose headers are scanned (they may be transitively included) but should not contribute public symbols. If any public API entity is defined in a private directory, a warning is printed to help catch accidental API exposure. |
+
+## Snapshot format
+
+The `.api` files use a minimal pseudo-C++ syntax designed for easy diffing:
+
+```
+namespace facebook::react {
+  class ComponentDescriptor {
+    public ComponentDescriptor(ComponentDescriptorParameters params);
+    public ComponentHandle getComponentHandle();
+  }
+
+  enum class AccessibilityRole {
+    None = 0,
+    Button = 1,
+  }
+}
+```
+
+- Scopes and members are sorted alphabetically.
+- Access specifiers (`public`, `protected`) are preserved.
+- Template parameters are included.
+- Doc comments and source file names are stripped.