AsDecided v0.23.0 - native cutover

The first native-only AsDecided release.

• Ships decided and decided-mcp as native Rust binaries.
• Retires the Python engine, Python CLI fallback, Explorer, and PyPI distribution.
• Uses rac-spec as the language-neutral compatibility authority.
• Preserves the former Python implementation at the immutable python-engine-final tag.

Python API consumers should use itsthelore/rac-sdk.