Abstract base class for integration (cross-service) events.
An empty semantic subclass of DomainEvent — no added fields, and no
framework reader: it is a semantic MARKER only. Nothing dispatches on
instanceof IntegrationEvent — the M14 messaging bridge selects forwarded
events by its configured eventTypes string list, never by class identity
— and the in-memory bus publishes it like any other event. A typed
cross-service contract (wire envelope, schema version, causal chain) is
defineIntegrationEvent in @setu-ts/messaging-plugin, not this class.