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/wipeout.wgsl

@@ -89,7 +89,12 @@ struct FamilyBatch {
 
 // ── Vertex shader ──────────────────────────────────────────────────────────
 
-struct VIn  { @location(0) pos: vec3<f32> }
+struct VIn  {
+    @location(0) pos: vec3<f32>,
+    @location(1) translation: vec2<f32>,
+    @location(2) translation_low: vec2<f32>,
+    @location(3) draw_depth: f32,
+}
 struct VOut {
     @builtin(position) clip: vec4<f32>,
     @location(0)       xz:   vec2<f32>,
@@ -107,17 +112,17 @@ struct VOut {
     // pattern math doesn't catastrophically cancel.
     // Double-single relative-to-eye: anchor high cancels eye_high (Sterbenz);
     // boundary-local v.pos + anchor low + (−eye_low) carry the residual.
-    let hi = vec3<f32>(h.origin.x - u.eye_high.x,
-                       h.origin.y - u.eye_high.y,
+    let hi = vec3<f32>(h.origin.x + v.translation.x - u.eye_high.x,
+                       h.origin.y + v.translation.y - u.eye_high.y,
                        -u.eye_high.z);
-    let lo = vec3<f32>(v.pos.x + h.origin_low.x - u.eye_low.x,
-                       v.pos.y + h.origin_low.y - u.eye_low.y,
+    let lo = vec3<f32>(v.pos.x + h.origin_low.x + v.translation_low.x - u.eye_low.x,
+                       v.pos.y + h.origin_low.y + v.translation_low.y - u.eye_low.y,
                        v.pos.z - u.eye_low.z);
     o.clip = u.view_rot * vec4<f32>(hi + lo, 1.0);
     // Draw-order bias (see wire.wgsl): the mask erases what draws BELOW its
     // entity and loses to what draws above — including its own frame wire,
     // which rides a +half-rank override.
-    o.clip.z = o.clip.z - h.draw_depth * 0.001 * o.clip.w;
+    o.clip.z = o.clip.z - (h.draw_depth + v.draw_depth) * 0.001 * o.clip.w;
     o.xz   = vec2<f32>(v.pos.x, v.pos.y);
     return o;
 }