Validates that the specification an instance is being specified by matches the
instance kind: a Service must be specified by a :serviceSpecification, a
Resource by a :resourceSpecification.
Runtime counterpart to the compile-time
Diffo.Provider.Extension.Verifiers.VerifySpecificationKind. The verifier guards
the statically-declared specification do type on consumer leaves; this guards
the specification associated at runtime via the specified_by argument —
covering generic instances (Provider.Instance and resource leaves with no spec
DSL) and respecify, which the verifier cannot see.
Declared on the Service / Resource fragments with the expected spec type, so
each kind carries its own constraint. A no-op on changes that do not set
specified_by. A missing/unreadable spec is left to other machinery — this
validation only rejects a present, wrong-kind specification.