Raised by the Divergence Detector (Mitigation 3b, spec §7/§16) under
check_purity: true.
Rollback re-executes events. A pure handle_event/3 must re-emit exactly
the events it emitted the first time. A mismatch means the model read a
wall-clock, used unseeded randomness, or otherwise depended on hidden state —
breaking determinism. Surfaced here instead of as silently-wrong output.