[lloydsmart]

Summary

• add deterministic regression coverage for archive extraction failure and repeated CHD verification failure
• verify custom LOGFILE handling and dry-run source-tree immutability
• exercise repeated parallel invocations with isolated temporary workspaces
• extend the chdman stub with selectable verification failures and attempt logging
• ensure file-only logging returns success when optional console mirroring is disabled

Why

The conversion state machine handles destructive cleanup and source deletion, so the remaining failure paths need explicit regression protection before the release and CLI work in #34.

The coverage audit found most scenarios from #33 were already exercised by recent changes. This PR fills the remaining gaps without introducing the optional main() refactor.

Validation

• Bash syntax checks for the production script and new test harness
• all 10 shell test scripts pass under WSL, including the new extraction, verification, logging, dry-run, and parallel scenarios
• git diff --check

Closes #33