Say a layer whole where the mesh reports it, and give tests their placement (issue 651)

The facts snapshot, the take preview and the settings history read a layer as it
was set; tests that place give the layer through LayerOf.

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

internal/catalogue/data_test.go

@@ -173,7 +173,7 @@ func TestTheBackupHoldersLinesAreDerivedFromTheData(t *testing.T) {
 			{"id":"meta","path":"${dir:meta}","class":"rebuildable"}]},
 		"resources":[{"id":"meta","type":"directory","mode":"0700"}]}`)
 	facts := map[string]string{"account-home": "/home/op"}
-	with := Rendering{Settings: SettingsBy{"media": {{From: "node", Values: map[string]any{AccessesSetting: map[string]any{"films": "/tank/films"}}}}}}
+	with := Rendering{Settings: SettingsBy{"media": {LayerOf("node", map[string]any{AccessesSetting: map[string]any{"films": "/tank/films"}})}}}
 	backup, _, err := seatContributions([]Manifest{pg, agent, media}, Manifest{}, BackupSeat+":"+BackupKindBackup, with, facts, nil)
 	if err != nil {
 		t.Fatal(err)