[shimwell]

Summary

• Download official OpenMC JEFF 3.3/4.0 HDF5 libraries (default 4.0), including release-specific ENDF/B photon supplements.
• Preserve download, extraction and cleanup options, with release-scoped directories below the destination parent.
• Simplify CLI declarations and consolidate offline tests into parameterized matrices: 379 → 285 added lines.
• Keep README documentation in the existing download table, with a neutron-temperature column for ENDF/B, JEFF and TENDL instead of a JEFF-only paragraph.

Validation

• 322 offline tests passed, 1 environment-dependent skip; CLI help and Python 3.8 syntax checks passed.
• No previously covered unchanged implementation lines lost coverage. The CLI matrix now exercises 44 additional combinations.
• Both official URLs previously passed live checks; full multi-GB libraries were not downloaded.
• README-only follow-up: checked table structure, GitHub rendering and unchanged surrounding sections. Temperatures verified against OpenMC and the TENDL 2019/2021 HDF5 documentation; no code or tests changed.

Based on main after #43. CI policy and the single explicitly untested URL are unchanged. All commits and this PR are attributed to shimwell.

[shimwell]

CI prerequisite: #43. Please merge that first; this PR will then be rebased and the shared CI/test changes removed from its feature diff. The new JEFF HDF5 URLs passed checks; existing external-host failures are documented separately in #43. Flat test organization updates are prepared locally. All commits remain attributed to shimwell.