feat: add asset handling plugin for esbuild-service using Metro's getAssetData()

[Copilot]

Implements the asset handling plugin for @rnx-kit/esbuild-service, completing the Metro → esbuild replacement story for production bundling. The plugin reuses Metro's own getAssetData() implementation for scale variant discovery, MD5 hashing, and image dimension reading — only the code generation step is reimplemented as a JS string instead of a Babel AST.

incubator/esbuild-service

• src/plugins/assets.ts — new reactNativeAssets esbuild plugin:
  • Intercepts imports of image/font/media files (all Metro-default asset extensions via DEFAULT_ASSET_EXTS)
  • Calls metro/src/Assets.getAssetData() directly when Metro is available; falls back to a minimal implementation otherwise
  • Emits module.exports = require(assetRegistryPath).registerAsset({...}) — identical output to metro-transform-worker's assetTransformer
  • getCollectedAssets() is attached as a non-enumerable property so esbuild's plugin-option validator doesn't reject it
• src/bundle.ts — wires in the assets plugin; after bundling, copies assets to assetsDest via @rnx-kit/metro-service/assets
• src/types.ts — adds assetsDest, assetCatalogDest, assetDataPlugins to BundleServiceOptions

const assetsPlugin = reactNativeAssets({ platform: "ios", projectRoot });

await esbuild.build({ plugins: [assetsPlugin], ... });

// CollectedAssets can be passed to saveAssets() to copy files to disk
const assets = assetsPlugin.getCollectedAssets();

packages/metro-service

• src/assets.ts — new file re-exporting saveAssets, getSaveAssetsPlugin, and related types
• package.json — adds "./assets" subpath export so esbuild-service can consume these utilities without importing from internal paths

Test plan

cd incubator/esbuild-service
yarn test

9 new tests in test/assets.test.ts cover: registerAsset code generation, asset collection, @2x/@3x scale variant discovery, custom assetRegistryPath, blocklisted properties (files, fileSystemLocation) excluded from output, and custom publicPath.

Original prompt

Analyze Metro bundler (https://github.com/facebook/metro) and outline which parts can be replaced by esbuild (https://github.com/evanw/esbuild). Which parts needs to be reimplemented, and how much code can be reused to reimplement said parts.

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