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/bundledJson.test.mjs

@@ -2,13 +2,14 @@ import { test } from 'node:test';
 import assert from 'node:assert/strict';
 import { loadBundledJson } from './bundledJson.js';
 
-const MARINE = new URL('./local_data/natural_earth/marine.json', import.meta.url);
-const importMarine = () =>
-  import('./local_data/natural_earth/marine.json', { with: { type: 'json' } });
+const MARINE = new URL(
+  './local_data/natural_earth/marine.json',
+  import.meta.url,
+);
 
-test('bundled JSON: a file URL uses the JSON import under Node', async (t) => {
+test('bundled JSON: a file URL is read from disk under Node', async (t) => {
   const fetchMock = t.mock.method(globalThis, 'fetch');
-  const pack = await loadBundledJson(MARINE, importMarine);
+  const pack = await loadBundledJson(MARINE);
   assert.ok(pack.features.some((ft) => ft.name === 'Gulf of Mexico'));
   assert.equal(fetchMock.mock.callCount(), 0);
 });
@@ -21,16 +22,62 @@ test('bundled JSON: a served URL is fetched as plain JSON', async (t) => {
       headers: { 'content-type': 'application/json' },
     });
   });
-  const url = new URL('http://localhost/src/data/local_data/natural_earth/marine.json');
-  const importJson = () => assert.fail('the browser path must not import JSON');
-  assert.deepEqual(await loadBundledJson(url, importJson), { features: [] });
+  const url = new URL(
+    'http://localhost/src/data/local_data/natural_earth/marine.json',
+  );
+  assert.deepEqual(await loadBundledJson(url), { features: [] });
   assert.deepEqual(requests, [url.href]);
 });
 
 test('bundled JSON: an HTTP error rejects so the retryable loader can retry', async (t) => {
-  t.mock.method(globalThis, 'fetch', async () => new Response('', { status: 404 }));
+  t.mock.method(
+    globalThis,
+    'fetch',
+    async () => new Response('', { status: 404 }),
+  );
   await assert.rejects(
-    loadBundledJson(new URL('http://localhost/missing.json'), importMarine),
+    loadBundledJson(new URL('http://localhost/missing.json')),
     /HTTP 404/,
   );
 });
+
+test('bundled JSON: an aborted load rejects before reading', async () => {
+  const controller = new AbortController();
+  controller.abort();
+  await assert.rejects(loadBundledJson(MARINE, { signal: controller.signal }), {
+    name: 'AbortError',
+  });
+});
+
+test('bundled JSON: no data pack is imported as a module', async () => {
+  // A literal JSON import in app code makes the production build emit a
+  // JavaScript copy of the pack beside the fetched JSON, which the browser
+  // never loads. Tests are not bundled, so they may still import JSON.
+  const { spawnSync } = await import('node:child_process');
+  const result = spawnSync(
+    'git',
+    [
+      'grep',
+      '-nE',
+      String.raw`import\(\s*['"][^'"]*local_data/[^'"]*\.json`,
+      '--',
+      'src',
+      ':!*.test.mjs',
+    ],
+    { cwd: new URL('../../', import.meta.url), encoding: 'utf8' },
+  );
+  // git grep exits 1 when nothing matches.
+  assert.equal(result.status, 1, result.stdout);
+});
+
+test('bundled JSON: an older Node says which version is needed', async (t) => {
+  // Node before 20.16 / 22.3 has no process.getBuiltinModule.
+  const original = process.getBuiltinModule;
+  t.after(() => {
+    process.getBuiltinModule = original;
+  });
+  process.getBuiltinModule = undefined;
+  await assert.rejects(loadBundledJson(MARINE), {
+    message: /needs Node 24\.14 or newer to read marine\.json/,
+  });
+});