MigrationStateEvent interface

Migration handshake event, Phase F — the propose. The SOURCE instance of raw.migrationId has answered "what state would you hand over?" with either raw.payload (opaque; empty is a valid acceptance) or a non-empty raw.refused, which ends the migration before any target has been placed. The refusal is on the envelope, so a coordinator never opens the blob to learn it was turned down.

Signature: