The mesh issues an assignment's subjects: a membership per module on a machine (hq ADR 0160)

For every module on every machine the controller composes what that instance serves — its machine's
address always, the module's plain address in a queue when it is alone or its definition says its
instances are interchangeable — the verbs of the seats it holds at the seats' subjects, where its
events land, and what it may reach, resolved the same way for the modules it invokes. Published
beside the node's declaration on `mesh.assignment.<node>.<module>`, last per subject in a stream
that allows direct reads, and the account may read exactly its own. Composed from the same records
the bus's accounts are, so what a runtime serves and what its account may are one composition.
`instances: interchangeable` is the one fact a definition states for it.

The shape issued is the shape the mesh already had, so nothing moves when the membership arrives;
the runtime that reads it instead of deriving it is the next piece.

internal/inventory/busrecords.go

@@ -49,7 +49,8 @@ func (i *Inventory) BusRecords(ctx context.Context) (broker.Records, error) {
 		}
 	}
 
-	out := broker.Records{Assigned: map[string][]broker.Declared{}, People: map[string][]string{}}
+	out := broker.Records{Assigned: map[string][]broker.Declared{}, People: map[string][]string{},
+		Interchangeable: map[string]bool{}}
 	for _, n := range nodes {
 		out.Nodes = append(out.Nodes, n.Name)
 		modules, err := i.Assigned(ctx, n.Name)
@@ -72,6 +73,9 @@ func (i *Inventory) BusRecords(ctx context.Context) (broker.Records, error) {
 						"be derived", module, n.Name)
 			}
 			out.Assigned[n.Name] = append(out.Assigned[n.Name], declaredFor(m, seats))
+			if m.Instances == catalogue.InstancesInterchangeable {
+				out.Interchangeable[m.Module] = true
+			}
 		}
 	}