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_test.go

@@ -15,10 +15,23 @@
 package checker
 
 import (
+	"context"
 	"encoding/json"
 	"testing"
 )
 
+// dummyRule is a minimal CheckRule used only by tests in this package.
+type dummyRule struct {
+	name string
+	desc string
+}
+
+func (r *dummyRule) Name() string        { return r.name }
+func (r *dummyRule) Description() string { return r.desc }
+func (r *dummyRule) Evaluate(ctx context.Context, obs ObservationGetter, opts CheckerOptions) []CheckState {
+	return []CheckState{{Status: StatusOK, Message: r.name + " passed"}}
+}
+
 func TestStatus_MarshalJSON(t *testing.T) {
 	tests := []struct {
 		status Status