move viewer code to ES modules under public/src, vendor libraries to public/vendor

Co-Authored-By: Claude Opus 5.5 (1M context) <noreply@anthropic.com>

tests/zipreader.test.js

-const test = require('node:test');
-const assert = require('node:assert/strict');
-const { makeZip } = require('./zip-util.js');
-const ZipReader = require('../public/zipreader.js');
-const CityGML = require('../public/citygml.js');
+import test from 'node:test';
+import assert from 'node:assert/strict';
+import { makeZip } from './zip-util.js';
+import * as ZipReader from '../public/src/loading/zipreader.js';
+import * as CityGML from '../public/src/loading/citygml.js';
 
 async function readAll(stream) {
   const chunks = [];