fix: restore legacy MACE imports with ASE 3.28

[tdpham2]

ASE 3.28 no longer exports ExpCellFilter from ase.constraints, causing the legacy MACE optimizer import to fail and matkit.mlip to omit run_opt_mace.

Prefer ase.filters.ExpCellFilter with a fallback to ase.constraints for older ASE releases. This restores the legacy Python and CLI entry points while retaining the existing filter, optimization settings, optional dependency behavior, and ase>=3.22 requirement.

Add seven isolated regression cases covering both ASE import layouts, missing MACE, all three optimization modes, and successful CLI output. Optimization tests use ASE's CPU EMT calculator through a stub MACE factory; they require no model downloads or GPU dependencies. Record the fix in the changelog.

Validation:

• Before the fix: five new regression cases failed; the legacy-layout and missing-MACE cases passed.
• On ASE 3.28.0: PYTHONPATH=src python -m pytest -q tests/test_mlip*.py tests/test_cli.py — 109 passed.
• Ruff lint and formatting checks passed for the changed Python files.
• git diff --check passed.

Older ASE import behavior is emulated in tests; real MACE/GPU execution was not exercised.