server: split HTTP scaffolding into its own subpackage

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.

checker/types.go

@@ -372,21 +372,6 @@ type CheckerDefinitionProvider interface {
 	Definition() *CheckerDefinition
 }
 
-// InteractiveRelatedProviders is an optional interface an interactive
-// ObservationProvider can co-implement to declare sibling providers whose
-// Collect the SDK runs in-process during /check. Their results are
-// exposed as RelatedObservations on ObservationGetter and ReportContext,
-// mirroring the cross-checker lineage a happyDomain host resolves.
-//
-// For each sibling the SDK seeds options from the primary and, when the
-// primary implements DiscoveryPublisher, writes its entries into any
-// sibling option tagged AutoFill == AutoFillDiscoveryEntries. Sibling
-// errors are logged and skipped so the primary result still reaches the
-// user.
-type InteractiveRelatedProviders interface {
-	RelatedProviders() []ObservationProvider
-}
-
 // CheckerDefinition is the complete definition of a checker, registered via init().
 type CheckerDefinition struct {
 	ID              string                      `json:"id"`