Document monorepo build setup (#48420)

Summary:

Adds a long overdue README for our newer monorepo build setup.

Changelog: [Internal]

Differential Revision: D67740763

scripts/build/README.md

@@ -0,0 +1,107 @@
+# scripts/build
+
+Shared build setup for the React Native monorepo.
+
+## Overview
+
+These scripts form the modern build setup for JavaScript ([Flow](https://flow.org/)) packages in `react-native`, exposed as `yarn build`.
+
+> [!Tip]
+> Generally, React Native maintainers do not need to run `yarn build`, as all packages will run from source during development. Please continue reading if you are adding/removing a package or modifying its build configuration.
+
+#### Key info
+
+- **Which packages are included?**
+  - Currently, only Node.js-targeting packages are included, configured in `config.js`.
+  - We don't yet include runtime packages (targeting Metro). These are instead transformed in user space via `@react-native/babel-preset`.
+- **When does the build run?**
+  - Packages are built in CI workflows — both for integration/E2E tests, and before publishing to npm.
+
+#### Limitations/quirks
+
+> [!Note]
+> **🚧 Work in progress!** This is not the final state for our monorepo build tooling. Unfortunately, our solution options are narrow due to integration requirements with Meta's codebase.
+
+- Running `yarn build` will mutate `package.json` files in place, resulting in a dirty Git working copy.
+- We make use of "wrapper files" (`.js` → `.js.flow`) for each package entry point, to enable running from source with zero config. To validate these, package entry points must be explicitly defined via `"exports"`.
+
+## Usage
+
+**💡 Reminder**: 99% of the time, there is no need to use `yarn build`, as all packages will run from source during development.
+
+Build commands are exposed as npm scripts at the repo root.
+
+```sh
+# Build all packages
+yarn build
+
+# Build a specific package
+yarn build dev-middleware
+
+# Clean build directories
+yarn clean
+```
+
+Once built, developing in the monorepo should continue to work — now using the compiled version of each package.
+
+> [!Warning]
+> **Build changes should not be committed**. Currently, `yarn build` will make changes to each `package.json` file, which should not be committed. This is validated in CI.
+
+## Configuration
+
+Monorepo packages must be opted in for build, configured in `config.js` (where build options are also documented).
+
+```js
+const buildConfig /*: BuildConfig */ = {
+  'packages': {
+    'dev-middleware': {
+      emitTypeScriptDefs: true,
+      target: 'node',
+    },
+    ...
+```
+
+#### Required package structure
+
+Opting a package into the `yarn build` setup requires a strict file layout. This is done to simplify config and to force consistency across the monorepo.
+
+```sh
+packages/
+  example-pkg/
+    src/             # All source files
+      index.js       # Entry point wrapper file (calls babel-register.js) (compiled away)
+      index.flow.js  # Entry point implementation in Flow
+      [other files]
+    index.js.flow    # Shim for the Flow typechecker
+    package.json     # Includes "exports" field, ideally only src/index.js
+```
+
+Notes:
+
+- The additional root `index.js.flow` shim is needed due to Flow itself not supporting Package Exports.
+- To minimize complexity, prefer only a single entry of `{".": "src/index.js"}` in `"exports"` for new packages.
+
+## Build behavior
+
+Running `yarn build` will compile each package following the below steps, depending on the configured `target` and other build options.
+
+- Create a `dist/` directory, replicating each source file under `src/`:
+  - For every `@flow` file, strip Flow annotations using [flow-api-extractor](https://www.npmjs.com/package/flow-api-translator).
+  - For every entry point in `"exports"`, remove the `.js` wrapper file and compile from the `.flow.js` source.
+- Rewrite each package `"exports"` target to map to the `dist/` directory location.
+- If configured, emit a Flow (`.js.flow`) or TypeScript (`.d.ts`) type definition file per source file, using [flow-api-extractor](https://www.npmjs.com/package/flow-api-translator).
+
+Together, this might look like the following:
+
+```sh
+packages/
+  example-pkg/
+    dist/
+      index.js       # Compiled source file (from index.flow.js)
+      index.js.flow  # Flow definition file
+      index.d.ts     # TypeScript definition file
+      [other transformed files]
+    package.json     # "src/index.js" export rewritten to "dist/index.js"
+```
+
+**Link**: [Example `dist/` output on npm](https://www.npmjs.com/package/@react-native/dev-middleware/v/0.76.5?activeTab=code).