Add npm distribution

[dunctk]

What changed

• adds a dependency-free npm launcher for npx whyhot
• bundles native Apple Silicon and Intel macOS binaries in the published tarball
• adds reproducible binary build and manual npm publishing workflows
• documents one-off and global npm installation

Why

Developers can run whyhot without first installing Rust or Cargo. The package performs no install-time downloads and has no lifecycle scripts or runtime dependencies.

Validation

• cargo fmt --check
• cargo test --offline --locked (11 tests)
• cargo clippy --offline --all-targets -- -D warnings
• native release builds for aarch64-apple-darwin and x86_64-apple-darwin
• both architecture binaries execute on Apple Silicon
• npm test
• npm pack --dry-run (673 KB package)
• clean global install from the generated npm tarball
• installed launcher --version and --once smoke tests