Add comprehensive test coverage for upgrade semantics:
- 12 Python unit tests for store migration (forward-only, downgrade
refusal, idempotent behavior) across migrate_to_latest, init_store,
and open_store_readonly
- Enhanced packaging upgrade test to verify all five acceptance
criteria: snapshot before migration, store not rebuilt, config
survival, timer/removal no-ops during upgrade
- RPM-specific test for config file preservation (noreplace conffile)
Co-authored-by: CommandCodeBot <noreply@commandcode.ai>
