feat(layers): add native state manager

Replace session-only layer snapshots with drawing-backed DWG and DXF states, exposed through the ribbon and LAYERSTATE commands.\n\nCloses #562

src/ui/window/mod.rs

@@ -1,5 +1,6 @@
 pub mod about;
 pub mod layout_manager;
+pub mod layer_state_manager;
 pub mod plot;
 pub mod plugin_manager;
 pub mod shortcuts;