fix(bgm): pass options arg to buildStart in unit test

Vite 8 (rolldown) changed buildStart to take (this, options), so the
test's one-arg call failed typecheck. Add an ambient declaration for the
fixture's virtual bgm modules so its entry typechecks in the editor.

packages/bgm/src/vite.unit.test.ts

@@ -80,7 +80,9 @@ describe('bgm vite plugin', () => {
     const plugin = bgm({ root: gamesRoot });
     const watched: string[] = [];
     const context = { addWatchFile: (file: string) => watched.push(file) };
-    (plugin.buildStart as Callable<typeof plugin.buildStart>).call(context);
+    // Vite 8 (rolldown) `buildStart` takes `(this, options)`; the plugin ignores
+    // the options, so pass a placeholder.
+    (plugin.buildStart as Callable<typeof plugin.buildStart>).call(context, {} as never);
 
     // Every def file (real + virtual code blocks) is watched so edits
     // trigger a re-collect.