[Sad Catapella B8][Notebook reliability] Failed folder preflight escapes the widget handler without clearing the prior checked state #101

Basis

Batch 8 at d7a538e56d41ac1093ba79adcad397c21b3dfd32. Confirmed missing exception/state cleanup branch, but actual user-visible triggering conditions depend on the exception source. No test or injected failure executed in this audit.

Evidence

• check_folder() lines 653–680 wraps preflight(...) in try/finally to restore button state but has no except path to show a readable failure, clear/revoke an earlier preflight, mapping, confirmations and run state, or display technical details.
• _folder_edited() lines 1056–1076 clears state only if the folder path changes, not if its contents changed but a same-path recheck fails.
• Run failure UI already catches unexpected exceptions and shows a readable technical report, demonstrating the established UI error-handling pattern. Existing notebook tests cover normal button restoration but not preflight raising.

Scenario / impact

After a successful check and confirmations, the same experiment's files or metadata become unavailable (e.g. transient storage error). The user clicks Check folder again; if preflight() raises an unanticipated OSError/RuntimeError, the exception escapes with a traceback, while the old checked result and UI confirmations remain displayed. run_experiment still rechecks inputs and may refuse to run, but the notebook shows stale readiness. This is UI integrity/recovery, not proven incorrect pixels or output.

Fix / acceptance

Add a fail-closed exception path: invalidate stale checked state and confirmations, restore the button and show type(exc).__name__ and actionable guidance in the folder card; append traceback to Technical details, avoiding silent fallback to old results. Preserve explicit nonexception validation issues from preflight. Regression: inject a failure on a same-folder recheck after successful confirmation, verify exception handling, cleared readiness, no write/run, and successful recovery after next valid recheck. No Numba value.