Add MLIP backends and GPU examples

[tdpham2]

• ## Summary

• Add runtime-selectable backends for direct ASE MACE, Rootstock, and NVIDIA ALCHEMI MACE.
• Add agent-free APIs and CLI commands for single-structure and batch execution.
• Support energy evaluation and fixed-cell geometry optimization.
• Write per-structure JSON results and batch manifests with isolated failure reporting.
• Add optional dependency groups and Polaris installation/smoke-test scripts.
• Add GPU examples for all three calculator backends.
• Align MatKit’s calculator interface with the capabilities being integrated in Unify MLIP calculator and execution backend configuration argonne-lcf/ChemGraph#179.

Benchmarking is intentionally deferred to a follow-up change.

Validation

• PYTHONPATH=src pytest -q — 177 passed, 1 skipped
• Ruff formatting and lint checks passed
• GPU example compilation and --help smoke test passed
• Polaris shell scripts passed syntax checks