fix: a coarse lot is a parcel, not a 700 m building

The unidentified slab on downtown San Francisco was the lot ladder's
own coarsest rung. Tower fill of 1.5–2.2 assembles a Salesforce-shaped
site from 40 m lots; on a 400 m lot it is a 600–880 m cube, rotated
with the district's street grid, downtown palette. The same pose on
the Bay Area board uses 40 m lots and does not show it.

Cap both axes at NEIGHBOURHOOD_LOT_METRES. Fine rungs are unchanged
because 40 m and 80 m already sit under the cap. The test that would
have named this on the first photograph now exists.

src/test/render/blocksLotLadder.test.ts

@@ -28,7 +28,13 @@ import test from "node:test";
 import * as THREE from "three";
 
 import { setReconcile } from "../../cities/reconcile.ts";
-import { createBlocks, detailLotMetres, LOT_BUDGET, updateBlocksDetail } from "../../engine/blocks.ts";
+import {
+  createBlocks,
+  detailLotMetres,
+  LOT_BUDGET,
+  NEIGHBOURHOOD_LOT_METRES,
+  updateBlocksDetail,
+} from "../../engine/blocks.ts";
 import type { City, District } from "../../engine/types.ts";
 import { World } from "../../engine/world.ts";
 
@@ -219,3 +225,41 @@ test("every rung is drawn from the same mesh, geometry and material", async () =
     assert.equal(blocks.boundingSphere, null, "a stale sphere would hide the city it was not built at");
   }
 });
+
+/**
+ * The largest building footprint currently packed, in scene units.
+ *
+ * Same extract as `smallestFootprint`: the first three entries of each 4×4
+ * instance matrix are the X-axis of the building, whose length is its width.
+ * Depth is the Z-axis at offsets 8, 9, 10.
+ */
+function largestFootprintM(mesh: THREE.InstancedMesh, metresPerUnit: number): number {
+  const m = mesh.instanceMatrix.array as Float32Array;
+  let largest = 0;
+  for (let i = 0; i < mesh.count; i++) {
+    const at = i * 16;
+    const width = Math.hypot(m[at]!, m[at + 1]!, m[at + 2]!);
+    const depth = Math.hypot(m[at + 8]!, m[at + 9]!, m[at + 10]!);
+    const span = Math.max(width, depth) * metresPerUnit;
+    if (span > largest) largest = span;
+  }
+  return largest;
+}
+
+test("a coarse lot is a parcel, not a 700 m building", async () => {
+  const world = await built();
+  const blocks = createBlocks(world);
+  /*
+   * Budget 100 forces the coarsest rung — 400 m lots on this fixture, the
+   * same rung the merged board picked over FiDi when the 700 m slab was
+   * photographed. Without the cap, tower fill of 1.5–2.2 on a 400 m lot is
+   * a 600–880 m cube. With it, nothing on the board is wider than a block.
+   */
+  packAt(world, blocks, 100);
+  assert.ok(blocks.count > 0, "the coarsest rung packed nothing");
+  const widest = largestFootprintM(blocks, world.metresPerUnit);
+  assert.ok(
+    widest <= NEIGHBOURHOOD_LOT_METRES + 1,
+    `a ${widest.toFixed(0)} m building on a coarse lot is the FiDi slab`,
+  );
+});