Durable workflow runtime transitions, cancellation, awaits, signals, recovery, and callbacks.
Semantics version 2 is the v1.2 lifecycle contract. New rows default to that
contract, while pre-v1.2 rows remain on an explicit compatibility path until a
v2 runtime transition writes the newer durable cause fields. Historical rows are
never silently reinterpreted as if they were created under the v1.2 contract.