scoring: normalize unsupported ZIP member read errors #174

Follow-up for PR #4 at a575437 under the 08:30Z acceptance standard.

• Greptile thread 4115046418: load_session() catches BadZipFile but an unsupported compression method raises NotImplementedError at archive.open(member). Convert member-read failures into the documented input error and exit code 2, with a regression test.

This fails closed before importing or writing a result. No false result, source-data loss, privacy leak, path escape, or realistic memory-growth consequence was established. Deferred rather than blocking this PR. Duplicate mapping is assessed separately as a blocker, not included in this follow-up.