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/catalogue/manifest.go

@@ -293,6 +293,12 @@ type Manifest struct {
 	// module claiming a seat answers what that seat's protocol promises (novox/hq ADR 0118).
 	Tools []string `json:"tools,omitempty"`
 
+	// Instances says whether this module's instances are the same anywhere — `interchangeable` —
+	// so a call that names no machine may be answered by any of them (novox/hq ADR 0160). A fact
+	// about the software, not about the bus: a stateless web tool says it; a database does not,
+	// and its instances are then each addressed by machine, never confused for one another.
+	Instances string `json:"instances,omitempty"`
+
 	// Invokes are the tools this module calls, each `<module>.<tool>` or a role's `seat:<seat>.<verb>`,
 	// or the single entry `*` for every tool on the mesh (novox/hq ADR 0152, ADR 0154).
 	//
@@ -1173,6 +1179,11 @@ func ParseManifest(raw []byte) (Manifest, error) {
 				m.Module, r))
 		}
 	}
+	if m.Instances != "" && m.Instances != InstancesInterchangeable {
+		problems = append(problems, fmt.Sprintf(
+			"%s says its instances are %q; the one word is %q, for a module that is the same on every machine",
+			m.Module, m.Instances, InstancesInterchangeable))
+	}
 	for _, offer := range m.Provides {
 		p := offer.Name
 		if !name.MatchString(p) {
@@ -1960,3 +1971,7 @@ func (o OwnSecrets) Paths() map[string]string {
 	}
 	return out
 }
+
+// InstancesInterchangeable is the one value of a definition's `instances`: the module is the same
+// on every machine, so any instance may answer for the module.
+const InstancesInterchangeable = "interchangeable"