docs: add CLAUDE.md (#2859)

CLAUDE.md

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+# NAPI-RS Project Guide
+
+## Project Structure
+
+**Core Architecture:**
+
+- `/crates/` - Rust implementation
+  - `napi/` - Main runtime library (Node-API bindings)
+  - `napi-sys/` - Low-level FFI bindings
+  - `macro/` - Procedural macros (`#[napi]` attributes)
+  - `backend/` - Code generation and TypeScript definitions
+  - `build/` - Build utilities
+- `/cli/` - Command-line tool (@napi-rs/cli)
+- `/examples/napi/` - Comprehensive test suite and examples
+- Monorepo using Cargo workspaces (Rust) + Yarn workspaces (JS)
+
+**Key Files:**
+
+- Root `Cargo.toml` - Workspace configuration
+- `/crates/backend/src/typegen.rs` - TypeScript generation logic
+- `/cli/src/utils/typegen.ts` - CLI TypeScript processing
+
+## Testing, Building, and Running Tests
+
+### Building
+
+```bash
+# Build all tests
+yarn build:tests
+
+# Build specific example (most common for testing changes)
+yarn workspace @examples/napi build
+```
+
+### Testing
+
+```bash
+# Run all tests in the example project
+yarn workspace @examples/napi test
+
+# Update test snapshots after changes
+yarn workspace @examples/napi test -u
+
+# Run specific test file
+yarn workspace @examples/napi test __tests__/values.spec.ts
+
+# Run Rust unit tests
+cargo test
+
+# Run linting
+yarn lint
+```
+
+### Common Development Workflow
+
+1. Make changes to Rust code
+2. Build: `yarn workspace @examples/napi build`
+3. Test: `yarn workspace @examples/napi test`
+4. Update snapshots if needed: `yarn workspace @examples/napi test -u`
+
+## Important Notes
+
+- JSDoc comments (`///`) in Rust become TypeScript documentation
+- Use `#[napi(object)]` for plain objects (interfaces)
+- Use `#[napi]` on impl blocks for classes
+- The `#[napi(js_name = "...")]` attribute renames in JS/TS
+- Build output: `.node` files in project root
+- TypeScript definitions: `index.d.cts`
+
+## Other notes
+
+- **Type generation**: Rebuild with `yarn workspace @examples/napi build` after changes
+- **Test snapshots**: Update with `-u` flag when output changes intentionally