Providers that implement the new CheckerInteractive interface
(RenderForm + ParseForm) get a built-in HTML form on GET /check and
a consolidated result page on POST /check that runs the standard
Collect -> Evaluate -> GetHTMLReport / ExtractMetrics pipeline. This
lets a checker be used directly from a browser outside of happyDomain,
with the checker itself resolving what the host would normally
auto-fill (typically via its own DNS queries).
Also guards NewServer against a nil Definition() so providers that
advertise CheckerDefinitionProvider without a ready definition no
longer panic at registration.
