Move server.go and interactive.go (and their tests) from the root
checker/ package into checker/server/. Plugin and builtin consumers of
the SDK now import only checker/ and no longer drag net/http,
html/template, or the form-rendering code into their artifacts: on
checker-dane.so this drops the binary by ~1.2 MB and removes 170
html/template symbols along with the net/http contribution that came
from the SDK itself.
Breaking for standalone consumers (main.go):
NewServer(p) -> server.New(p)
CheckerInteractive -> server.Interactive
InteractiveRelatedProviders -> server.Siblings
Providers that only satisfy the interactive interfaces structurally
(method set match, no explicit type reference) need no source change;
only main.go has to switch its import path and the constructor name.
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.
