enhancement: implement list and verify for 7z format

[bug-ops]

Summary

exarch list and exarch verify return an error for 7z archives even though extraction works:

Error: invalid archive: 7z archive listing not yet supported

Reproduction

exarch list archive.7z        # Error: 7z archive listing not yet supported
exarch verify archive.7z      # Error: 7z archive listing not yet supported
exarch extract archive.7z ./  # Works correctly

Expected behavior

list and verify should support 7z archives consistently with TAR and ZIP.

sevenz-rust2 (already a dependency) supports iterating entries via SevenZReader — the same reader used in extraction. The listing implementation should reuse that path.

[bug-ops]

Additional gap found (session 004): exarch create file.7z ... fails with 7z archive creation not yet supported. The 7z format support is extraction-only. list, verify, and create all need implementation.