BpmnMigrationBlocker : One reason a (from_version, to_version) pair is not eligible for an identity migration (#922). rule is the discriminant; rule_number groups it (1 superset – every v1 element id, sequence-flow id and message name exists in v2; 2 stable meaning – an id that survives must still denote the same kind, scope, process, message binding and (for a boundary event) host activity; 3 join stability – no element’s set of incoming sequence flows may change, because JoinState.expected is derived once and never refreshed, so a stale one wedges the instance with no error at all). The remaining properties are the rule’s own, and which are present depends on rule.