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/scene/model/mod.rs

@@ -3,6 +3,7 @@ pub mod gdi;
 pub mod hatch_model;
 pub mod hatch_patterns;
 pub mod image_model;
+pub mod instance_model;
 pub mod material_model;
 pub mod ole_pres;
 pub mod pdf_raster;