License under GPLv3

Add SPDX headers to the source, a License section to the README, the
OFL notice for the bundled fonts, and ship LICENSE in the Docker image.

webtest/city.test.mjs

@@ -1,3 +1,5 @@
+// SPDX-License-Identifier: GPL-3.0-or-later
+// Copyright (C) 2026 Andy Dixon
 import { test } from 'node:test';
 import assert from 'node:assert/strict';
 import { layoutCity, buildingHeight, spatialIndex, rayBox, pickRay, CITY } from '../web/city.js';