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/middleware.ts

@@ -0,0 +1,22 @@
+import { NextResponse } from 'next/server';
+import type { NextRequest } from 'next/server';
+
+export function middleware(request: NextRequest) {
+  const { pathname } = request.nextUrl;
+
+  // Only protect /dashboard
+  if (pathname.startsWith('/dashboard')) {
+    const session = request.cookies.get('arcrun_session');
+    if (!session?.value) {
+      const loginUrl = new URL('/login', request.url);
+      loginUrl.searchParams.set('redirect', pathname);
+      return NextResponse.redirect(loginUrl);
+    }
+  }
+
+  return NextResponse.next();
+}
+
+export const config = {
+  matcher: ['/dashboard/:path*'],
+};