20.16Error-handling reviewer

For every error handler and retry:

Classify the possible failure as:
- transient
- permanent data/business
- authentication/configuration
- unknown/ambiguous outcome

Check:
- Is retry appropriate?
- Is operation idempotent?
- Can prior attempt already have committed?
- What happens after exhaustion?
- Is error normalized to public API?
- Could On Error Continue accidentally hide failure?

Very high-value automated review.