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/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)
+	}
+}