Each source's Query() method previously set r.Listed and r.Severity,
embedding verdict logic inside the prober. Evaluation now lives in a
dedicated Evaluate(SourceResult) (bool, string) method per source,
keeping Query() as pure observation.
A package-level EvaluateResult() helper looks up the source by ID and
delegates to its Evaluate method; rules.go, report.go, types.go, and
provider.go all call this instead of reading pre-set r.Listed/r.Severity
values. An unknownSource sentinel handles results whose source is no
longer registered.
