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/app/view/mod.rs

@@ -1571,6 +1571,7 @@ impl OpenCADStudio {
             PluginManager => (940, 600),
             UpdateNotice => (560, 460),
             Layers => (900, 360),
+            LayerStateManager => (720, 420),
             Plot => (760, 540),
             LayoutManager => (640, 320),
             Plotstyle => (780, 540),