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.go

@@ -99,6 +99,11 @@ func Settle(base map[string]any, layers []Layer) (map[string]any, error) {
 // overridden lays settings over a map whose keys are its contract: a contribution, a served fact.
 // Only the keys the map already declares are touched; the rest of a layer is somebody else's
 // business (a file's, another destination's) and is left to reach it there.
+//
+// A declared value may itself be the operator's, `${setting:<key>}` (ADR 0155): a mail provider
+// serves its domain, an identity provider its issuer, and neither is the definition's to state.
+// Filled from the layers after the overrides, and refused by name when nothing sets it — a literal
+// placeholder handed to a consumer is a service configured against a string nobody meant.
 func overridden(base map[string]any, layers []Layer, what string) (map[string]any, error) {
 	kept := make([]Layer, 0, len(layers))
 	for _, layer := range layers {
@@ -110,7 +115,28 @@ func overridden(base map[string]any, layers []Layer, what string) (map[string]an
 		}
 		kept = append(kept, Layer{From: layer.From, Values: values})
 	}
-	return settle(base, kept, nil, what)
+	merged, err := settle(base, kept, nil, what)
+	if err != nil {
+		return nil, err
+	}
+	for key, value := range merged {
+		s, ok := value.(string)
+		if !ok {
+			continue
+		}
+		for _, asked := range settingsUsed(s) {
+			v, set := settingValue(layers, asked)
+			if !set {
+				return nil, fmt.Errorf(
+					"%s says ${setting:%s} for %q, and nothing sets %q — an operator's value is the "+
+						"assignment's, never the definition's (novox/hq ADR 0112)%s",
+					what, asked, key, asked, orNoSettings(layers))
+			}
+			s = strings.ReplaceAll(s, "${setting:"+asked+"}", plainly(v))
+		}
+		merged[key] = s
+	}
+	return merged, nil
 }
 
 // settle lays the layers over a module's own values, in order.