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/shaders/hatch.wgsl

@@ -97,6 +97,10 @@ struct LineFamily {
 struct VIn {
     @location(0) local_xy:       vec2<f32>,
     @location(1) instance_index: u32,
+    @location(2) translation: vec2<f32>,
+    @location(3) translation_low: vec2<f32>,
+    @location(4) draw_depth: f32,
+    @location(5) visible: u32,
 }
 
 struct VOut {
@@ -114,7 +118,7 @@ struct VOut {
     // frustum-culls the primitive and no fragment runs. (WGSL
     // forbids literal NaN so this out-of-NDC trick replaces the
     // usual NaN-degenerate-triangle.)
-    if visibility[v.instance_index] == 0u {
+    if visibility[v.instance_index] == 0u || v.visible == 0u {
         o.clip = vec4<f32>(2.0, 2.0, 2.0, 1.0);
         o.xz = vec2<f32>(0.0, 0.0);
         o.instance_index = v.instance_index;
@@ -126,14 +130,15 @@ struct VOut {
     // against eye_high (Sterbenz); local + anchor low + (−eye_low) carry the
     // residual. `local` is small (boundary-relative), so adding it in the low
     // term keeps full precision at UTM-scale anchors.
-    let hi = vec3<f32>(inst.world_origin.x - u.eye_high.x,
-                       inst.world_origin.y - u.eye_high.y,
+    let hi = vec3<f32>(inst.world_origin.x + v.translation.x - u.eye_high.x,
+                       inst.world_origin.y + v.translation.y - u.eye_high.y,
                        -u.eye_high.z);
-    let lo = vec3<f32>(local.x + inst.world_origin_low.x - u.eye_low.x,
-                       local.y + inst.world_origin_low.y - u.eye_low.y,
+    let lo = vec3<f32>(local.x + inst.world_origin_low.x + v.translation_low.x - u.eye_low.x,
+                       local.y + inst.world_origin_low.y + v.translation_low.y - u.eye_low.y,
                        -u.eye_low.z);
     o.clip = u.view_rot * vec4<f32>(hi + lo, 1.0);
-    o.clip.z = o.clip.z - inst.draw_depth * DRAW_ORDER_BIAS * o.clip.w;
+    o.clip.z = o.clip.z
+        - (inst.draw_depth + v.draw_depth) * DRAW_ORDER_BIAS * o.clip.w;
     o.xz = local;
     o.instance_index = v.instance_index;
     return o;