Fill a preference's ${setting:} from its manifest default (hq ADR 0262)

Without a default, a running module could never gain a setting: the file asking for it
failed to compose until set, and the key was refused as stray until a file asked for it.
Defaults sit under the mesh's and the node's settings, never merge into a JSON file, are
refused for the operator's own values, and settings shows each value's source.

cmd/mesh-controller/modules.go

@@ -502,13 +502,38 @@ func settingsCommand(ctx context.Context, args []string) error {
 		}
 		if !has {
 			fmt.Printf("%s on %s: no layer — the module's definition says\n", positionals[0], where)
-			return nil
+		} else {
+			shown, err := json.MarshalIndent(values, "", "  ")
+			if err != nil {
+				return err
+			}
+			fmt.Println(string(shown))
 		}
-		shown, err := json.MarshalIndent(values, "", "  ")
+		// Every value the module gives a default or a layer sets, and where it came from (novox/hq
+		// ADR 0262): the default, the mesh's layer, or this node's. Said after the layer, which stays
+		// the first thing printed because a caller reads it before replacing it (ADR 0217).
+		known, err := inv.Catalogue(ctx)
 		if err != nil {
 			return err
 		}
-		fmt.Println(string(shown))
+		m, ok := known[positionals[0]]
+		if !ok || len(m.Settings) == 0 {
+			return nil
+		}
+		var layers []catalogue.Layer
+		if *node != "" {
+			if mesh, has, err := inv.Layer(ctx, "", positionals[0]); err != nil {
+				return err
+			} else if has {
+				layers = append(layers, catalogue.Layer{From: catalogue.MeshWideLayer, Values: mesh})
+			}
+			if has {
+				layers = append(layers, catalogue.Layer{From: *node, Values: values})
+			}
+		} else if has {
+			layers = append(layers, catalogue.Layer{From: catalogue.MeshWideLayer, Values: values})
+		}
+		fmt.Print(describeEffective(positionals[0], where, catalogue.Effective(m, layers)))
 		return nil
 
 	case "clear":
@@ -526,6 +551,23 @@ func settingsCommand(ctx context.Context, args []string) error {
 	}
 }
 
+// describeEffective says each setting's value on a machine or the whole mesh, where it came from, and
+// the module's default when a layer overrides it.
+func describeEffective(module, where string, values []catalogue.SettingSource) string {
+	var b strings.Builder
+	fmt.Fprintf(&b, "%s on %s, every value and where it comes from:\n", module, where)
+	for _, v := range values {
+		value, _ := json.Marshal(v.Value)
+		fmt.Fprintf(&b, "  %s = %s  (%s", v.Key, value, v.From)
+		if v.HasDefault && v.From != catalogue.DefaultLayer {
+			d, _ := json.Marshal(v.Default)
+			fmt.Fprintf(&b, "; the default is %s", d)
+		}
+		b.WriteString(")\n")
+	}
+	return b.String()
+}
+
 // nodeFlag is ` --node <node>` for a machine's layer, nothing for the whole mesh's.
 func nodeFlag(node string) string {
 	if node == "" {

cmd/mesh-controller/plan.go

@@ -1425,7 +1425,7 @@ func servedOnNode(ctx context.Context, inv *inventory.Inventory, node string,
 	}
 	serves := catalogue.ServedOn(m, provision, ports)
 	if len(serves) > 0 {
-		serves, err = catalogue.Settle(serves, layers)
+		serves, err = catalogue.Settle(serves, catalogue.WithDefaults(m, layers))
 		if err != nil {
 			return nil, err
 		}

cmd/mesh-controller/settings_effective_test.go

@@ -0,0 +1,21 @@
+package main
+
+import (
+	"testing"
+
+	"github.com/novox/mesh-controller/internal/catalogue"
+)
+
+// `settings` says each value and where it came from, and the default a layer overrides (novox/hq ADR 0262).
+func TestSettingsSayWhereEachValueComesFrom(t *testing.T) {
+	got := describeEffective("dunst", "laptop", []catalogue.SettingSource{
+		{Key: "font-size", Value: float64(13), From: "laptop", Default: float64(10), HasDefault: true},
+		{Key: "width", Value: float64(250), From: catalogue.DefaultLayer, Default: float64(250), HasDefault: true},
+	})
+	want := "dunst on laptop, every value and where it comes from:\n" +
+		"  font-size = 13  (laptop; the default is 10)\n" +
+		"  width = 250  (default)\n"
+	if got != want {
+		t.Fatalf("said\n%s\nwant\n%s", got, want)
+	}
+}