A served value or a contribution's may be the operator's: ${setting:…} fills there too, refused by name when unset

Issue 173's rule needs it: a mail provider serves its domain and an identity provider its issuer,
and neither is the definition's to state. Declared as ${setting:<key>}, filled from the layers after
the overrides; a key so asked for is not stray.

internal/catalogue/settings_test.go

@@ -213,3 +213,24 @@ func TestContentThatIsNotJSONIsRefusedWhereSomebodyIsLooking(t *testing.T) {
 		t.Fatal("a module claiming to merge as JSON shipped something else and was accepted")
 	}
 }
+
+func TestAServedValueMayBeTheOperators(t *testing.T) {
+	// A mail provider serves its domain and an identity provider its issuer; neither is the
+	// definition's to state (ADR 0155). Filled from the layers, refused by name when unset.
+	served, err := Settle(map[string]any{"port": 587, "domain": "${setting:domain}"},
+		[]Layer{{From: "the mesh", Values: map[string]any{"domain": "example.tld"}}})
+	if err != nil {
+		t.Fatal(err)
+	}
+	if served["domain"] != "example.tld" {
+		t.Errorf("the operator's value did not fill the served key: %v", served)
+	}
+	_, err = Settle(map[string]any{"domain": "${setting:domain}"}, nil)
+	if err == nil || !strings.Contains(err.Error(), `"domain"`) {
+		t.Errorf("a served value nothing sets must be refused by name; got %v", err)
+	}
+	m := Manifest{Module: "mail", Serves: map[string]map[string]any{"smtp": {"domain": "${setting:domain}"}}}
+	if unused := UnusedSettings(m, []Layer{{From: "the mesh", Values: map[string]any{"domain": "x"}}}); len(unused) != 0 {
+		t.Errorf("a setting a served fact asks for is not stray: %v", unused)
+	}
+}