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>

landing/app/globals.css

@@ -0,0 +1,22 @@
+@import "tailwindcss";
+
+:root {
+  --background: #0a0a0a;
+  --foreground: #ededed;
+}
+
+@theme inline {
+  --color-background: var(--background);
+  --color-foreground: var(--foreground);
+}
+
+body {
+  background: var(--background);
+  color: var(--foreground);
+  font-family: -apple-system, BlinkMacSystemFont, 'Segoe UI', Roboto, 'Helvetica Neue', Arial, sans-serif;
+  -webkit-font-smoothing: antialiased;
+}
+
+pre, code {
+  font-family: 'JetBrains Mono', 'Fira Code', Consolas, monospace;
+}