Read placement from a layer's Placement, never from its config values (issue 651)

ADR 0348 clause 11: places, accesses and kept networks are placement, not config.
Today's mergeable files still take them as said, so nothing composes differently.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

internal/catalogue/setting_defaults.go

@@ -207,7 +207,8 @@ func Effective(m Manifest, layers []Layer) []SettingSource {
 			Default: d.Default, HasDefault: true}
 	}
 	for _, layer := range layers {
-		for key, v := range layer.Values {
+		// As the layer is said, placement too: `settings` answers it as a person set it.
+		for key, v := range layer.Whole() {
 			s, ok := byKey[key]
 			if !ok {
 				s = &SettingSource{Key: key}