Add an opt-in InteractiveRelatedProviders interface: a checker served
by /check can declare sibling ObservationProviders the SDK will run
in-process after the primary Collect. The SDK auto-fills any sibling
option tagged AutoFill==AutoFillDiscoveryEntries from the primary's
DiscoverEntries output, mirroring the host's AutoFill wiring so
siblings that work in-host work here too. Results are exposed through
ObservationGetter.GetRelated and ReportContext.Related, unblocking
cross-checker rules (e.g. DANE reading checker-tls probes) on the
standalone /check flow.
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.
