One bus: the AMQP transport is gone from the host

The mesh runs on the seat's bus alone (novox/hq ADR 0131, design 28 task 5.5). The host's old
dialling and enrolment paths are deleted with the switch that chose between them; a membership or
a token naming another bus is refused before anything is sent, rather than dialled on a transport
that no longer exists.

internal/link/hearing.go

@@ -2,15 +2,15 @@ package link
 
 import (
 	"context"
+	"fmt"
 	"time"
 )
 
 // What a host hears, as the host's own words for it.
 //
-// The outbound half went behind `Bus` (bus.go) and a node's two statements stopped naming a
-// transport. This is the other half — dialling, and the declarations that arrive — and it is where
-// the transport reached furthest: the run loop selected on a channel of the client library's own
-// delivery type, so every part of holding a node in its mesh knew which bus it was on.
+// The outbound half is behind `Bus` (bus.go); this is the other half — dialling, and the
+// declarations that arrive. The run loop reads its own words for a declaration rather than the
+// client library's delivery type, so nothing past this file knows what carried it.
 //
 // **The host still imports nothing of the mesh's own** (novox/hq ADR 0005). This is its own
 // interface over its own libraries, and it agrees with the controller only because a conformance
@@ -18,8 +18,8 @@ import (
 
 // Link is this node's live connection to its mesh: what it hears, and what it says.
 //
-// One interface rather than two, because **dialling is where the transport is chosen** and choosing
-// it twice is how one half of a node ends up on a different bus from the other.
+// One interface rather than two, because dialling once is what keeps both halves of a node on the
+// same connection.
 type Link interface {
 	// Bus is what this node says: what it applied, and that it is here.
 	Bus
@@ -45,9 +45,8 @@ type Link interface {
 // because applying is reconciliation: it converges rather than repeating.
 //
 // There is one way of being done rather than two. A declaration set aside because a newer arrived
-// with it is settled exactly as an applied one is, on both buses, and the difference between them
-// is a fact the *report* carries — a second method here would be a distinction the transport does
-// not make.
+// with it is settled exactly as an applied one is, and the difference between them is a fact the
+// *report* carries — a second method here would be a distinction the bus does not make.
 type Declaration interface {
 	// Body is the signed declaration as it arrived, bytes unchanged: a node verifies what it
 	// received rather than what it re-encoded.
@@ -62,23 +61,15 @@ type Declaration interface {
 // Named Open rather than Dial because Dial is this package's raw TLS dial, which the enrolment path
 // uses to see a certificate before it trusts anything.
 //
-// **Both transports ship and this is the one place that chooses** (novox/hq ADR 0116: nothing moves
-// a node's bus before step 5). Until then every membership names the bus the mesh runs on today,
-// and the rollout is this switch and the credential behind it — not a change anywhere in the loop
-// that reads from what comes back.
+// **The mesh has one bus** (novox/hq ADR 0131): the one the broker seat delivers. A membership
+// still records which transport it was minted for, so a host can say what it is dialling, and a
+// membership recorded for anything else is a membership this host cannot use.
 func Open(ctx context.Context, m Membership, timeout time.Duration) (Link, error) {
-	switch m.Transport {
-	case OnNATS:
-		return dialNats(ctx, m, timeout)
-	default:
-		return dialCurrent(ctx, m, timeout)
+	if m.Transport != OnNATS {
+		return nil, fmt.Errorf("this membership is for %q, and the mesh's bus is %s", m.Transport, OnNATS)
 	}
+	return dialNats(ctx, m, timeout)
 }
 
-// The buses a node can be on. Empty is the one the mesh runs on today, which is every node until
-// the rollout — so a membership recorded before any of this existed reads as correct rather than as
-// unset.
-const (
-	OnCurrent = ""
-	OnNATS    = "nats"
-)
+// OnNATS is the bus a membership names: the mesh's own, and the only one.
+const OnNATS = "nats"