[shimwell]

Summary

• Unify JEFF 3.2/3.3/4.0 ACE conversion under convert_jeff (default 4.0), using current NEA sources.
• Remove the old release-specific modules and entry points; select a release with -r.
• Preserve temperature/material validation, in-memory metastable correction, staged HDF5/XML output and bounded downloads. JEFF 4.0 thermal data is the public 12-case/316-table subset, not the full TSL library.

Testing scope

• Follow the repository standard: no test-file changes, new test dependencies, new jobs or new workflow inputs in this PR.
• Existing CLI smoke checks and the manual processing recipe use convert_jeff instead of the removed commands. The manual recipe still processes just one neutron temperature.
• Expanded feature and downloader tests are deferred to a follow-up PR. No production code changed during this test-scope reduction.

Validation

• Existing offline suite: 238 passed, 1 environment-dependent skip (OpenMC is installed), in 0.87 seconds locally.
• Unified CLI help and workflow configuration checks passed. No full-library processing was run for this revision.

The existing offline/manual CI boundary and explicitly untested IAEA URL are unchanged. Shared download fixes remain identical to #44. All commits and this PR are attributed to shimwell.

[shimwell]

CI prerequisite: #43. Please merge that first; this PR will then be rebased onto the shared deterministic CI/test setup. The current processing failure is the unrelated full FENDL download returning HTTP 403 after a shared utility change. JEFF conversion/offline tests and all JEFF ACE URL checks passed. Flat test organization updates are prepared locally. All commits remain attributed to shimwell.