0192 — ExceptionClaim payload unmarshal failure is fatal, not retried

Context. When DrainExceptions claims a recorded exception, the payload from message_log is unmarshaled again before the handler runs. That unmarshal could in principle fail — should it get its own retry/dead-letter path?

Decision. ExceptionClaim’s json.Unmarshal failure returns the raw error, treated as fatal.

Consequences. The payload already deserialized successfully once, in CursorClaim, to reach the exception path at all — and message_log rows are immutable — so a failure here can only mean an invariant broke elsewhere. Surfacing it loudly beats building recovery machinery for an unreachable case. Rejected: a dedicated retry/dead-letter path for unmarshal errors — unreachable by construction, and it would hide corruption instead of reporting it.