perf: reduce mobile rendering and cull offscreen characters and building tiles

tests/buildings.test.ts

@@ -51,7 +51,8 @@ test('downloaded footprints are deduplicated, closed, and generate finite merged
   const city=createBuildings(data,terrain);
   assert.equal(Object.values(city.counts).reduce((a,b)=>a+b,0),data.features.length);
   assert.ok(city.counts.tagged>0 && city.counts.levels>0 && city.counts.fallback>0);
-  assert.ok(city.group.children.length<=3);
+  assert.ok(city.group.children.length > 3, 'separate city districts can be culled independently');
+  assert.ok(city.group.children.length < 100, 'batch buildings instead of one draw call per building');
   for(const mesh of city.group.children as Mesh[]) assert.ok(Array.from(mesh.geometry.attributes.position.array).every(Number.isFinite));
   city.dispose();
 });