router/realm: retype broker / dealer fields to Broker / Dealer interfaces

Replaces the concrete *broker / *dealer pointer fields on the realm
struct with the Broker / Dealer interface types extracted in the
previous commit. The newRealm() constructor signature changes to
accept the interfaces too, but the call sites still pass concrete
*broker / *dealer values — Go's structural typing handles the
implicit satisfaction.

Behavior-preserving: the only thing that changes is the static type
of two struct fields. Coverage unchanged (63.2%).

Step 2 of 3 in the Broker/Dealer interface extraction (gammazero#315).

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

Author: Marko Petzold

router/realm.go

@@ -39,8 +39,8 @@ type testamentBucket struct {
 type realm struct {
 	uri wamp.URI
 
-	broker *broker
-	dealer *dealer
+	broker Broker
+	dealer Dealer
 
 	authorizer Authorizer
 
@@ -94,7 +94,7 @@ var (
 )
 
 // newRealm creates a new realm with the given RealmConfig, broker and dealer.
-func newRealm(config *RealmConfig, broker *broker, dealer *dealer, logger stdlog.StdLog, debug bool) (*realm, error) {
+func newRealm(config *RealmConfig, broker Broker, dealer Dealer, logger stdlog.StdLog, debug bool) (*realm, error) {
 	if !config.URI.ValidURI(config.StrictURI, "") {
 		return nil, fmt.Errorf(
 			"invalid realm URI %v (URI strict checking %v)", config.URI, config.StrictURI)