feat(bgm): add vite plugin and build integration test

Move the bgm vite plugin into the package so it can be tested in
isolation from the web app. Serve each package as a JSON module,
serializing its maps, and verify resolution through a real vite build
against a self-contained fixture.

packages/bgm/src/index.ts

@@ -3,4 +3,5 @@ export * from './schemas.js';
 export * from './markdown.js';
 export * from './parse.js';
 export * from './variants.js';
-export * from './collect.js';
+export * from './collect.js';
+export * from './vite.js';