Marker interface for all query plans.
Concrete plan classes live in Protocol\Planning\ (an outer ring that Service\ must not import). This marker exists solely so that the resolver interfaces in Service\Contracts\ can carry a type-checked parameter without creating an inward dependency.
Callers in Protocol\ receive the resolver from RuntimeSchemaInterface and safely pass the concrete plan — the runtime type is always correct because SchemaBuilder only pairs entity-set resolvers with entity-set plans.