Reactor step: SPARQL behavioral differential evaluation.
When two or more %AshR2RML.SPARQL.Observation{} structs are present in
:observations, calls AshR2RML.SPARQL.Differential.compare/3 to verify
parity across execution strategies. Returns {:ok, nil} when insufficient
observations are supplied (< 2).
This step is non-retryable (max_retries 0 in the pipeline DSL).
Arguments
:observations— list of%AshR2RML.SPARQL.Observation{}structs (may be empty or nil).:metadata— optional map forwarded to the differential evaluator.