perf(render): instance block geometry

Share source buffers across block placements and render nested content with GPU instance transforms.\n\nChunk silhouettes and compact mesh material data so large drawings stay under device buffer limits and remain responsive.

src/modules/view/plot_window.rs

@@ -59,6 +59,7 @@ impl CadCommand for PlotWindowCommand {
             depth_override: None,
             fill_is_3d: false,
             fill_is_2d_solid: false,
+            render_instance: None,
             pick_tris: Vec::new(),
             pick_tris_low: Vec::new(),
             dash_from_start: false,

src/modules/view/zoom_window.rs

@@ -78,6 +78,7 @@ impl CadCommand for ZoomWindowCommand {
             depth_override: None,
             fill_is_3d: false,
             fill_is_2d_solid: false,
+            render_instance: None,
             pick_tris: Vec::new(),
             pick_tris_low: Vec::new(),
             dash_from_start: false,