Validation + SHACL-AF inference execution (Layers 3, 6, 7).
Layer 3 lives here: the naive denotational evaluator that is the conformance
oracle — relational path evaluation ([path]), value-type checks
([value]), and shape/schema satisfaction ([validate]). The rule/fixpoint
inference engine (Layer 6) and compiled executors (Layer 7) come later; every
execution mode must agree with this oracle.