Describe the bus on its own terms

Comments framed the new bus by what it replaces — a comparison in almost
every explanation, which reads as though NATS were a variant of the old
thing rather than the mesh's nervous system. Removed throughout, and
OverAMQP becomes OverCurrent: the seam's two sides are the bus the mesh
runs on today and the one being built, not two protocols.

What remains is the client library's own package name, which is its name.

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, link.OverAMQP{Channel: channel}, link.KeyModuleBuilt, "builder", on, announced); err != nil {
+		if err := link.EmitEvent(publishCtx, link.OverCurrent{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.