CAT-REG: distribution's references name the seat as their registry part

The reach came from MESH_REGISTRY_REACH, which nothing sets, so every published event
answered /<name>@<digest>. The store answers the path; the holder puts the seat's name
before it, as catalogue.InRegistrySeat reads it back (ADR 0605 §9; lead's ruling on A3n).

Claude-Session: https://claude.ai/code/session_01JG73KJNrH5EgFXMKKYSrig

modules/distribution/cmd/distribution-tools/main.go

@@ -473,7 +473,7 @@ func tools(v *holder.Verbs, windows kept[holder.Window], callers func() (holder.
 
 func main() {
 	base, _ := url.Parse("http://127.0.0.1:5001")
-	store := &holder.OCI{Base: base, Registry: os.Getenv("MESH_REGISTRY_REACH"), Upstream: &holder.Upstream{}}
+	store := &holder.OCI{Base: base, Upstream: &holder.Upstream{}}
 	h := &holder.Holder{Objects: parts.BusObjects(holder.IncomingObjects), Builder: delivery.BuilderVersions{Ask: stdio.Ask}, Store: store, Say: say, Finding: finding,
 		Transfers: parts.BusTransfers[holder.Opening](holder.TransfersState)}
 	v := &holder.Verbs{Holder: h, Catalog: store, Holds: kept[holder.Hold]{state: "holds", key: holder.Hold.Key}, Mirrors: mirrors{}}