tests: add non-proprietary demo CIF fixtures so CI can run the full suite

[bobleesj]

Problem

Many tests under tests/ expect CIF files under tests/data/cif/** (e.g. URhIn.cif, radius/ICSD/MS/COD samples, ensemble fixtures). Those files are not in the repo (gitignored / never committed as permanent public fixtures), so matrix CI fails with FileNotFoundError even though the library code is fine.

We intentionally do not want proprietary / licensed database dumps (ICSD, PCD, etc.) in a public GitHub repo.

Goal

Add a small, redistributable demo CIF set sufficient for CI:

• Prefer structures we already ship or can freely redistribute (e.g. under src/cifkit/data/cifs/ such as GdSb.cif / HoSb.cif, or other open COD/public-domain CIFs).
• Wire tests (or a thin fixture layer) to those demos or keep heavy golden tests behind a local-only path / marker (pytest -m proprietary skipped on CI).
• Document in README/CONTRIBUTING: what CI covers vs what needs private local data.

Out of scope

• No algorithm / geometry / CN logic changes required for this issue.
• Do not upload proprietary CIF databases.

Context

Related migration (bobleesj.utils → in-tree modules) did not change core compute logic; it moved elemental-data helpers and deps. CI red is a fixture packaging problem.

Acceptance

• PR tests / matrix can run without missing-file errors on a clean clone
• Demo CIFs are clearly non-proprietary / allowed to publish
• Optional: mark or skip tests that still need private data

[bobleesj]

Downstream SAF tracking (same theme, no proprietary uploads): bobleesj/structure-analyzer-featurizer#77