Stage 0.2: Cellpy-file HDF5 characterization tests

[jepegit]

Summary

• Add tests/test_cellpy_file_roundtrip.py — Stage 0.2 characterization suite locking current cellpy-file (HDF5) load/save behavior before the module extraction refactor.
• Commit fid-populated v8 oracle testdata/hdf5/20160805_test001_45_cc_v8_with_fids.h5 and compare helpers in tests/cellpy_file_support.py.
• Three tests marked @pytest.mark.essential: v8 round-trip (full frame equality), limits-prefix trap (unprefixed limits in /CellpyData/info), and selector={"max_cycle": N} truncation.
• Full suite also pins legacy v6/v7 loads, v4/v5 TypeError (current behavior), v0 WrongFileVersion, missing HDF key exception, and check_file_ids smoke.

Closes #429

Test plan

• uv run pytest tests/test_cellpy_file_roundtrip.py -v (10 passed)
• uv run pytest -m essential --ignore=tests/test_plotutils_summary_plot.py (21 passed)
• CI essential (linux / uv) green on PR

Notes

• v4/v5 fixtures currently fail with TypeError on _extract_meta_from_old_cellpy_file_max_v7 (missing upgrade_from_to) — tests pin that behavior for the refactor to fix deliberately.
• graphify-out/ local rebuild left unstaged (not part of this PR).

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