v0.5.1: the Python runtime is published as localizer

The PyPI name localizer was handed over by the maintainer of the unrelated project that held it, so the Python runtime is now published under it: pip install localizer (import name unchanged). localizer-py, the name of the first release, stays on PyPI as a metadata-only alias that depends on localizer, so existing installs and lock files keep working. Docs, README and AGENTS.md updated; the Go module and the Action are unchanged from v0.5.0.