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/hatch_model.rs

@@ -155,6 +155,7 @@ pub const GPU_BOUNDARY_SEP: f32 = 1.0e30;
 /// A hatched region defined by a closed polygon boundary.
 #[derive(Clone, Debug)]
 pub struct HatchModel {
+    pub render_instance: Option<super::instance_model::RenderInstance>,
     /// World XY anchor (in the same offset-relative coordinate space as
     /// the rest of the scene — `world_offset` already subtracted, but
     /// kept at f64 precision). Boundary vertices are stored as f32