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/collect.test.ts

@@ -3,7 +3,7 @@ import * as path from 'node:path';
 import { fileURLToPath } from 'node:url';
 import { loadDefs, collectPackages } from './collect.js';
 
-const fixtureRoot = path.resolve(path.dirname(fileURLToPath(import.meta.url)), '__fixtures__');
+const fixtureRoot = path.resolve(path.dirname(fileURLToPath(import.meta.url)), '__fixtures__', 'harbor');
 
 describe('collectPackages', () => {
   it('collects the harbor package from markdown code blocks', () => {