fix(layer): keep auto-registered layer names verbatim; one sync helper

ensure_layer had started trimming the name, so an entity on " A" got a
table entry "A" it never resolves to. Register the name as given again
and drop the unused return value. The five copies of the dirty-flag check
become sync_registered_layers.

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

src/scene/entity.rs

@@ -143,17 +143,16 @@ impl Scene {
     /// layer 0. Auto-registering keeps it on its own layer (#252). Names are
     /// registered verbatim so the writer's (case-insensitive) lookup matches;
     /// the always-present default layer "0" and empty names are no-ops.
-    pub fn ensure_layer(&mut self, name: &str) -> bool {
-        let trimmed = name.trim();
-        if trimmed.is_empty() || trimmed == "0" || self.document.layers.contains(trimmed) {
-            return false;
+    /// Flags `layer_table_dirty` so the app refreshes its layer lists.
+    pub fn ensure_layer(&mut self, name: &str) {
+        if name.trim().is_empty() || self.document.layers.contains(name) {
+            return;
         }
-        let mut layer = codec::tables::Layer::new(trimmed);
+        let mut layer = codec::tables::Layer::new(name);
         layer.handle = self.document.allocate_handle();
         let _ = self.document.layers.add(layer);
-        self.invalidate_layer_dependencies(&[trimmed.to_string()]);
+        self.invalidate_layer_dependencies(&[name.to_string()]);
         self.layer_table_dirty = true;
-        true
     }
 
     pub(crate) fn ensure_app_id(&mut self, name: &str) {