feat: add landing page + builtins Worker + BETA_TEST guide + README

- landing/: Next.js 15 app for arcrun.dev (dashboard, integrations,
  API docs, login). Deploys via Cloudflare Pages — CI scan skips
  this via pages_build_output_dir marker.
- builtins/: minimal Hono Worker at arcrun-builtins (/init for
  one-shot component registry seeding). initComponents logic is
  flagged stale in src/index.ts for future rewrite.
- BETA_TEST.md: pre-launch validation playbook.
- README.md: updated to match current arcrun.dev / acr CLI flow.

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

builtins/src/index.ts

@@ -0,0 +1,27 @@
+// u6u-builtins Worker
+// 所有零件已遷移至 u6u-core/registry/components/ 的 TinyGo .wasm 版本
+// 此 Worker 保留 /init 端點供初始化 Component Registry 使用
+
+import { Hono } from 'hono';
+import { cors } from 'hono/cors';
+import type { Bindings } from './types';
+import { initComponents } from './actions/initComponents';
+
+const app = new Hono<{ Bindings: Bindings }>();
+app.use('*', cors());
+
+app.get('/', c => c.json({
+  service: 'u6u-builtins',
+  version: '2.0.0',
+  status: 'ok',
+  note: '所有零件已遷移至 WASM，請使用 Component Registry',
+}));
+
+// POST /init — 把所有零件上架到 Component Registry（冪等，可重複執行）
+app.post('/init', async c => {
+  const result = await initComponents(c.env);
+  const allOk = result.failed === 0;
+  return c.json({ success: allOk, ...result }, allOk ? 200 : 207);
+});
+
+export default app;