Name system seats by scope; let a module define its own (ADR 0121)

System seats are mesh-* (one, mesh-wide) or node-* (one per node). Renamed:
the-build-machine -> mesh-build-machine (+scope mesh), the-catalogue ->
mesh-catalog, the-dns-port -> node-dns-resolver, the-intrusion-prevention ->
node-intrusion-prevention, the-packet-filter -> node-packet-filter,
the-resolver-configuration -> node-resolver-config, the-uplink -> node-uplink.
Removed the-showcase from the set — it becomes the first module-defined seat.

A manifest may declare its own seats (DefinesSeats); a claim is a system seat,
a reserved mesh-*/node-* name the mesh does not define (refused), or a
module-defined seat valid only when the manifest declares it.

Deferred: the delivering registry seats (git, npm-package-registry,
the-artifact-store) and the-private-network (a scope + server/client model
change), per ADR 0121.

internal/catalogue/resolver_manifests_test.go

@@ -170,7 +170,7 @@ func TestTwoThingsDecidingWhatAMachineAsksAreRefused(t *testing.T) {
 	if err == nil {
 		t.Fatal("resolv-conf and resolved-split-dns were both assigned to one machine")
 	}
-	if !strings.Contains(err.Error(), "the-resolver-configuration") {
+	if !strings.Contains(err.Error(), "node-resolver-config") {
 		t.Fatalf("the refusal does not say what was claimed: %v", err)
 	}
 }