Typed result returned by every stage function

Stages return this instead of throwing on failure so a struggling stage cannot silently emit valid-looking output.

Slots

status

"success" or "failure"

value

annotated StateTransitionData on success; NULL on failure

reason

diagnostic string on failure; "" on success

provenance

list of ProvenanceStep records emitted by this stage; the same records are appended to value@provenance on success