feat(engine)!: retire Python engine and PyPI distribution

[tcballard]

Summary

• retire the Python engine, Python CLI/SDK surface, Explorer, ingest implementation, parity harness, and Python test suite from rac-core
• remove pyproject.toml, PyPI/TestPyPI/shim publishing, and all pip install rac-core action paths
• make Rust the only product engine and normal CLI/MCP runtime
• move the artifact registry and evaluation corpus into native-engine paths
• replace wheel releases and the Python container with native archives, native GitHub Actions, and a Rust container
• point Python API consumers to itsthelore/rac-sdk
• document the immutable python-engine-final archive tag

Verification

• cargo check --workspace --locked
• cargo test --workspace --release
• cargo clippy --workspace --all-targets --release -- -D warnings
• vendored artifact registry matches rac-spec
• Rust reproduced all 11 rac-spec output-parity cases
• live corpus invariants passed: validity, determinism, cache/no-cache equality, freshness after mutation
• decided validate decisions
• decided relationships decisions --validate
• decided gate decisions --sarif
• workflow and composite-action YAML parsed successfully

Compatibility boundary

The former Python implementation is preserved for historical review at the
immutable python-engine-final tag. It is no longer built, shipped, tested, or
used as a live oracle.