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/naturalEarthRegions.js

@@ -124,17 +124,9 @@ let _entries = null;
 const PACKS = {
   regions: {
     url: new URL('./local_data/natural_earth/regions.json', import.meta.url),
-    importJson: () =>
-      import('./local_data/natural_earth/regions.json', {
-        with: { type: 'json' },
-      }),
   },
   marine: {
     url: new URL('./local_data/natural_earth/marine.json', import.meta.url),
-    importJson: () =>
-      import('./local_data/natural_earth/marine.json', {
-        with: { type: 'json' },
-      }),
   },
 };
 
@@ -178,8 +170,8 @@ function buildEntries(pack, kind) {
  */
 const loadIndex = createRetryableLoader(async () => {
   const [regions, marine] = await Promise.all([
-    loadBundledJson(PACKS.regions.url, PACKS.regions.importJson),
-    loadBundledJson(PACKS.marine.url, PACKS.marine.importJson),
+    loadBundledJson(PACKS.regions.url),
+    loadBundledJson(PACKS.marine.url),
   ]);
   _entries = [
     ...buildEntries(regions, 'natural'),