The controller's outbound link behind a seam, with both transports

Step 3.4, first half. Every one of these took an *amqp.Channel, so the
transport reached every caller and swapping it meant touching all of them.
The seam turned out to be small — the controller sends exactly two kinds of
message that expect no answer — which is the same measurement that said
this bus could be replaced at all.

Bus is stated in the mesh's words, not a transport's: PublishEvent and
PublishDeclaration. Two implementations, both shipping, because steps 1 to
4 leave every node on AMQP and the NATS one is selected at the rollout.
Both ship is also what makes them comparable: one conformance fixture holds
both to the same envelope, and the NATS one is checked against a real
server reading back from the stream rather than from the code that wrote it.

Still on *amqp.Channel: RequestBuild and Ask, which carry reply-queue
machinery, and the whole consume side — the control loop, enrolment, serve.

internal/link/declare.go

@@ -5,8 +5,6 @@ import (
 	"encoding/json"
 	"fmt"
 	"time"
-
-	amqp "github.com/rabbitmq/amqp091-go"
 )
 
 // Signer is whatever holds the control plane's signing key.
@@ -21,7 +19,7 @@ type Signer interface {
 // something else, and the node would refuse a declaration that was genuinely the mesh's.
 //
 // Published to the node's own queue, which its account alone may read.
-func Declare(ctx context.Context, channel *amqp.Channel, signer Signer, node string,
+func Declare(ctx context.Context, bus Bus, signer Signer, node string,
 	declaration []byte, timeout time.Duration) error {
 
 	if !json.Valid(declaration) {
@@ -41,13 +39,5 @@ func Declare(ctx context.Context, channel *amqp.Channel, signer Signer, node str
 	publish, cancel := context.WithTimeout(ctx, timeout)
 	defer cancel()
 
-	// Published to the queue directly rather than through the exchange: a declaration is for one
-	// node, and routing it by name through a shared exchange would mean a binding per node that
-	// nothing removes when a node is retired.
-	return channel.PublishWithContext(publish, "", QueueFor(node), false, false,
-		amqp.Publishing{
-			ContentType:  "application/json",
-			DeliveryMode: amqp.Persistent,
-			Body:         body,
-		})
+	return bus.PublishDeclaration(publish, node, body)
 }