Tell a module where a mesh seat's holder is reached (hq ADR 0222)

The container runtime's module must state the mesh's registry to the runtime it owns, so the
controller can stop writing that into the runtime's file (hq issue 190). ${seat:<seat>:reach}
answers host:port without a binding: nothing required, granted or minted, and the address is
one the mesh already composes into every reference it built. Only mesh-artifact-store is
answered; another seat is refused by name. Unanswered in a file written into as JSON, the empty
member is dropped, so the runtime is never told to trust "".

cmd/mesh-controller/plan.go

@@ -732,6 +732,13 @@ func renderingFor(ctx context.Context, open *stores, node string,
 	if err != nil {
 		return catalogue.Rendering{}, inventory.Node{}, err
 	}
+	// Where this machine reaches each mesh seat's holder, for ${seat:<seat>:reach} (novox/hq ADR
+	// 0222): the artifact store as this network reaches it, which the container runtime is told to
+	// trust (ADR 0082). The same address composed into every reference the mesh built.
+	var reach map[string]string
+	if artifactStore != "" {
+		reach = map[string]string{"mesh-artifact-store": artifactStore}
+	}
 	return catalogue.Rendering{
 		BusMembership: memberships[node],
 		Settings:      settings, Generators: gens, Grants: grants, Needed: needed, Ports: ports,
@@ -739,7 +746,7 @@ func renderingFor(ctx context.Context, open *stores, node string,
 		Machines: machines, Zones: zones,
 		Suffix: overlay.Suffix(), MeshRange: meshRange, TunnelInterface: overlay.Interface, Accounts: accounts, Foundation: foundation,
 		Kept: kept, Adopted: record.Adopted, OutwardLinks: outwardLinks,
-		Given: given, Taken: taken, Seats: seats, ArtifactStore: artifactStore, Built: built,
+		Given: given, Taken: taken, Seats: seats, ArtifactStore: artifactStore, SeatReach: reach, Built: built,
 		BusUsers: busUsers,
 	}, record, nil
 }