Fix LLC center-mask fallback and fixture schema

[jder]

Summary

• align the tiny LLC test fixture with the live LLC4320 root's complete variable/coordinate set, dimension layouts, and mask dtypes
• preserve mask_c as the preferred center mask while falling back to hFacC when mask_c is absent
• add regression coverage for the production schema and the unresolved hFacC fallback from PR Add LLC CPU data canonicalization #670

Root cause

LLC canonicalization hard-coded mask_c in its retained-variable whitelist. The test fixture omitted hFacC, so it could not expose that stores containing only the standard MITgcm center-cell fraction would drop their only center wet mask before canonicalization.

The live LLC4320 root contains both mask_c and hFacC; the fixture now models both accurately, including fractional float32 hFac* arrays.

Validation

• uv run pytest tests/test_utils_data.py -k llc — 7 passed
• uv run pytest tests/test_config.py tests/test_datasets.py -k llc — 9 passed
• pre-commit Ruff and mypy hooks passed; Ruff formatting was applied
• the remaining schema hook was interrupted on the slow shared host; GitHub CI should complete the full suite

Review context: #670 (comment)