docs(core): add documentation for dual ESM/CJS build pattern

Add reference to ESM/CJS build documentation in the main README and
create a detailed guide explaining how to implement dual builds for
modules requiring browser support, especially those using wasm-utxo.

Issue: BTC-2936

Co-authored-by: llm-git <llm-git@ttll.de>

Author: OttoAllmendinger

CLAUDE.md

@@ -65,6 +65,11 @@ yarn compile
 # Build for development
 yarn compile-dbg
 ```
+
+## Dual ESM/CJS Builds
+
+For modules that need browser support (especially those using `@bitgo/wasm-utxo`), see [docs/esm.md](docs/esm.md) for the dual ESM/CJS build pattern.
+
 ## Code Style
 
 ### Commits

docs/esm.md

@@ -0,0 +1,97 @@
+# Dual ESM/CJS Module Pattern
+
+Modules that need to work in the browser (especially those importing `@bitgo/wasm-utxo`) must provide both ESM and CJS builds. The browser webpack build uses ESM to properly handle async WASM initialization.
+
+## When to Use
+
+Add dual builds to a module if:
+- It imports `@bitgo/wasm-utxo` (required for browser WASM support)
+- It's imported by other modules that need browser support
+- It needs to be bundled for browser use
+
+## How to Convert a Module
+
+### 1. Update package.json
+
+```json
+{
+  "main": "./dist/cjs/src/index.js",
+  "module": "./dist/esm/index.js",
+  "browser": "./dist/esm/index.js",
+  "types": "./dist/cjs/src/index.d.ts",
+  "exports": {
+    ".": {
+      "import": {
+        "types": "./dist/esm/index.d.ts",
+        "default": "./dist/esm/index.js"
+      },
+      "require": {
+        "types": "./dist/cjs/src/index.d.ts",
+        "default": "./dist/cjs/src/index.js"
+      }
+    }
+  },
+  "files": ["dist/cjs", "dist/esm"],
+  "scripts": {
+    "build": "npm run build:cjs && npm run build:esm",
+    "build:cjs": "yarn tsc --build --incremental --verbose .",
+    "build:esm": "yarn tsc --project tsconfig.esm.json"
+  }
+}
+```
+
+### 2. Update tsconfig.json
+
+Change output directory to `dist/cjs`:
+
+```json
+{
+  "compilerOptions": {
+    "outDir": "./dist/cjs",
+    "rootDir": "."
+  }
+}
+```
+
+### 3. Create tsconfig.esm.json
+
+```json
+{
+  "extends": "./tsconfig.json",
+  "compilerOptions": {
+    "outDir": "./dist/esm",
+    "rootDir": "./src",
+    "module": "ES2020",
+    "target": "ES2020",
+    "moduleResolution": "bundler",
+    "lib": ["ES2020", "DOM"],
+    "declaration": true,
+    "declarationMap": true,
+    "skipLibCheck": true
+  },
+  "include": ["src/**/*"],
+  "exclude": ["node_modules", "test", "dist"],
+  "references": []
+}
+```
+
+### 4. Add Webpack Alias (if needed for browser bundle)
+
+If your module is imported in the browser bundle and uses `@bitgo/wasm-utxo`, you must add a webpack alias in [`webpack/bitgojs.config.js`](../webpack/bitgojs.config.js):
+
+```javascript
+resolve: {
+  alias: {
+    '@bitgo/your-module': path.resolve('../your-module/dist/esm/index.js'),
+  }
+}
+```
+
+**Why aliases are required:** Webpack doesn't automatically use the `browser` or `module` fields when resolving imports from CJS code. While we could use `resolve.conditionNames: ['browser', 'import', ...]` to prioritize ESM, this breaks third-party packages like `@solana/spl-token` and `@bufbuild/protobuf` that have broken ESM builds. The explicit aliases target only our packages.
+
+## Reference Implementations
+
+- [`modules/utxo-core`](../modules/utxo-core) - Full dual build setup
+- [`modules/abstract-utxo`](../modules/abstract-utxo) - Full dual build setup
+- [`modules/utxo-staking`](../modules/utxo-staking) - Full dual build setup
+- [`modules/utxo-ord`](../modules/utxo-ord) - Full dual build setup