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.

cmd/mesh-builder/main.go

@@ -268,7 +268,7 @@ func answer(ctx context.Context, channel *amqp.Channel, publisher builder.Publis
 			"manifest": json.RawMessage(result.Manifest), "against": result.Against,
 			"made": result.Made,
 		}
-		if err := link.EmitEvent(publishCtx, channel, link.KeyModuleBuilt, "builder", on, announced); err != nil {
+		if err := link.EmitEvent(publishCtx, link.OverAMQP{Channel: channel}, link.KeyModuleBuilt, "builder", on, announced); err != nil {
 			// Said, not fatal: the build happened and was answered. A module the catalogue has not
 			// heard of is a gap somebody can close; a build reported as failed because announcing
 			// it failed is a lie about work that was done.