build: ship each bundled data pack once

loadBundledJson's callers passed a literal JSON import as a Node fallback,
which made the production build emit a JavaScript copy of every pack beside
the JSON the browser fetches. The loader now reads file: URLs itself under
Node (through process.getBuiltinModule, synchronously, as promptly as the
cached import it replaces), and the callers pass only the URL. On a Node too
old to provide getBuiltinModule it says that Node 24.14 or newer is needed.
dist/ drops from 54 MB to 42 MB. A test keeps app code from importing a pack
as a module.

Co-Authored-By: Claude Opus 5.5 <[email protected]>

src/data/neighborhoodPolygons.js

@@ -22,10 +22,6 @@ const CITY_FILES = [
       './local_data/neighborhoods/san-francisco.json',
       import.meta.url,
     ),
-    importJson: () =>
-      import('./local_data/neighborhoods/san-francisco.json', {
-        with: { type: 'json' },
-      }),
   },
 ];
 
@@ -112,7 +108,7 @@ function cityLoader(city) {
   let loader = _cityLoaders.get(city.id);
   if (!loader) {
     loader = createRetryableLoader(async () => {
-      const fc = await loadBundledJson(city.url, city.importJson);
+      const fc = await loadBundledJson(city.url);
       return Array.isArray(fc.features) ? fc.features : [];
     });
     _cityLoaders.set(city.id, loader);