Add symmetry-graft module inspection and expansion

[jgassens]

Summary

• Add first-party symmetry_graft module under src/foldfoundry/modules/symmetry_graft/.
• Register symmetry and graft through ModuleContext.cli; root cli.py only loads the module host.
• Add Gemmi/NumPy/Biotite as symmetry-graft optional/dev dependencies, plus mmCIF/PDB inspection, Kabsch utilities, biological assembly expansion, docs, fixtures, and tests.

2MS2 smoke

• foldforge modules info symmetry_graft reports the module loaded with graft, symmetry CLI groups.
• foldforge symmetry inspect runs/2ms2_assembly_smoke/2MS2.cif detected biological assembly, NCS, crystal metadata, and deposited fallback.
• foldforge symmetry expand ... --assembly-id 1 wrote 183,900 atoms across 180 chains and the generated mmCIF re-parsed successfully.

Verification

• uv run pytest tests/test_module_discovery.py tests/test_modules_cli.py tests/test_symmetry_inspect.py tests/test_grafting_fit.py tests/test_assembly_expansion.py -q
• uv run pytest
• uv run ruff check .
• uv run ruff format --check .
• uv run mypy src