feat(bgm): add board game manifest loader

Parse yaml/json/toml and markdown code blocks into packages, expanding
$variants via typed-csv and collecting parts, surfaces, and setups by
include patterns. Ships zod validation, a vitest config, and 16 tests.

packages/bgm/src/index.ts

@@ -0,0 +1,6 @@
+export * from './types.js';
+export * from './schemas.js';
+export * from './markdown.js';
+export * from './parse.js';
+export * from './variants.js';
+export * from './collect.js';