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/wrangler.toml

@@ -0,0 +1,25 @@
+name = "arcrun-builtins"
+main = "src/index.ts"
+compatibility_date = "2025-02-19"
+compatibility_flags = ["nodejs_compat"]
+workers_dev = true
+
+# TODO: initComponents 需改寫 — 現在用 Service Binding + HTTP URL 模式上架零件
+# 應改為讀 registry/components/*/contract.yaml + .wasm，透過 POST /submit 上架
+# 或直接整合進 registry Worker，不再需要獨立 builtins Worker
+
+# KV Store：cron: 前綴 = 排程定義，ai-transform: 前綴 = 編譯後轉換函式
+[[kv_namespaces]]
+binding = "U6U_STORE"
+id = "a259bfcfbb7c43d28ce03fe2de36ac33"
+
+# Workers AI：ai-transform 零件的 compile 階段使用
+[ai]
+binding = "AI"
+
+# Cron Trigger：每分鐘掃描並執行到期的排程工作流
+[triggers]
+crons = ["* * * * *"]
+
+[vars]
+ENVIRONMENT = "development"