feat: cylinder occluders (shape/radiusCm/heightCm) for round props like the test rig post

public/js/exhibit.js

@@ -390,9 +390,21 @@ function buildOcclusion(scene) {
   occlusionGroup.clear();
   const mat = new THREE.MeshBasicMaterial({ colorWrite: false }); // depth-only
   for (const b of scene.buildings || []) {
-    const geo = new THREE.BoxGeometry(b.size[0], b.size[1], b.size[2]);
+    /* Two occluder shapes. Boxes cover LEGO buildings; cylinders exist for round
+     * props (the test rig's 85mm post), where a box silhouette would wrongly
+     * hide ghosts at the corners. Positions are floor-level, so each mesh is
+     * lifted by half its height. */
+    let geo, height;
+    if (b.shape === 'cylinder') {
+      const r = b.radiusCm ?? (b.size ? b.size[0] / 2 : 5);
+      height = b.heightCm ?? (b.size ? b.size[1] : 10);
+      geo = new THREE.CylinderGeometry(r, r, height, 24);
+    } else {
+      height = b.size[1];
+      geo = new THREE.BoxGeometry(b.size[0], b.size[1], b.size[2]);
+    }
     const mesh = new THREE.Mesh(geo, mat);
-    mesh.position.set(b.position[0], b.position[1] + b.size[1] / 2, b.position[2]);
+    mesh.position.set(b.position[0], b.position[1] + height / 2, b.position[2]);
     mesh.rotation.y = THREE.MathUtils.degToRad(b.yawDeg || 0);
     mesh.renderOrder = -1;
     occlusionGroup.add(mesh);