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/tsconfig.json

@@ -0,0 +1,12 @@
+{
+  "compilerOptions": {
+    "target": "ES2022",
+    "module": "ES2022",
+    "moduleResolution": "bundler",
+    "lib": ["ES2022"],
+    "types": ["@cloudflare/workers-types"],
+    "strict": true,
+    "noEmit": true
+  },
+  "include": ["src/**/*"]
+}