A specialized "harden" loop. Systematically enumerate and close edge cases and failure modes after the happy path works — boundaries, nulls, empty/overflow inputs, races, and error paths — so the robust cases get covered, not just the demo. The doer is cohardener. Use after cobuild/cotest pass the happy path, when a surface needs to survive hostile or unusual input before shipping. Optional, on-demand loop; runs standalone or after the core loop. Best on Sonnet (structured enumeration).
