- New stdlib module `plugin` with 7 methods: load_dir, load, list, has, call, each, each_if
- Plugins are namespaces compiled via build-lib, loaded at runtime via source
- Dynamic dispatch: plugin.call(name, func) -> ${name}__${func}
- Empty function bodies now emit `:` (bash no-op) instead of invalid empty block
- 21 new tests in test_plugin.py