Settings: older settings files split layers above the original (#2545)

Create Split Layers Above Original Layer became on by default in #2083, but a settings
file saved before then still holds `false` (every later save keeps it), so Split Layer
kept stacking the new layer below. Settings version 4 moves a stored `false` to the new
default once, like version 3 did for the old disk cache size; a choice made since stays.

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

crates/engine/src/tests/settings.rs

@@ -79,6 +79,15 @@ fn prefs_migrate_old_disk_cache_default() {
     assert_eq!(Prefs::from_json(r#"{"version": 3, "disk": {"diskCacheMaxGb": 100}}"#).disk.disk_cache_max_gb, 100);
 }
 
+#[test]
+fn prefs_migrate_old_split_layers_default() {
+    // Settings saved before #2083 kept "below"; they move to After Effects' "above" once (#2545).
+    let above = |doc: &str| Prefs::from_json(doc).general.create_split_layers_above;
+    assert!(above(r#"{"version": 3, "general": {"createSplitLayersAbove": false}}"#));
+    assert!(above(r#"{"version": 2, "general": {"createSplitLayersAbove": false}}"#));
+    assert!(!above(r#"{"version": 4, "general": {"createSplitLayersAbove": false}}"#), "a choice made since stays");
+}
+
 #[test]
 fn prefs_set_validates_keys_types_and_ranges() {
     let mut p = Prefs::default();