feat(render): M2 — React 渲染内核 + 布局画廊(playground)

- core 新增 render 子路径(vibe-registry/render):PageView + WidgetView + LayoutGallery
- layout 注册项支持 render 实现;缺省兜底 DefaultLayout
- widget 注册项 component 挂真实 React 组件
- PageBlockDecl 增加 slot 字段(块落在布局哪个槽位)
- playground:admin 布局真实骨架 + 组件 + 订单页,dev 预览 + 生产构建通过
- 全量回归:core/qa-orchestrator/playground typecheck 全绿

examples/playground/index.html

@@ -0,0 +1,16 @@
+<!doctype html>
+<html lang="zh-CN">
+  <head>
+    <meta charset="UTF-8" />
+    <meta name="viewport" content="width=device-width, initial-scale=1.0" />
+    <title>vibe-registry playground</title>
+    <style>
+      body { margin: 0; background: #fff; }
+      * { box-sizing: border-box; }
+    </style>
+  </head>
+  <body>
+    <div id="root"></div>
+    <script type="module" src="/src/main.tsx"></script>
+  </body>
+</html>