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/__fixtures__/vite-build/src/vite-env.d.ts

@@ -0,0 +1,18 @@
+/// <reference types="vite/client" />
+
+/**
+ * Ambient types for the virtual `bgm` modules served by the bgm vite plugin,
+ * so the fixture's entry typechecks in the editor (the plugin provides them at
+ * build time). Mirrors `apps/web/src/vite-env.d.ts`.
+ */
+declare module 'virtual:bgm/packages' {
+  import type { SerializedPackage } from '@tts/bgm';
+  const packages: SerializedPackage[];
+  export default packages;
+}
+
+declare module 'virtual:bgm/package/*' {
+  import type { SerializedPackage } from '@tts/bgm';
+  const pkg: SerializedPackage;
+  export default pkg;
+}