feat: Robotex landing describes architecture v2

- how it works: async POST /v2/tasks + polling, window held between tasks,
  /v1/solve stays synchronous; example uses an http proxy and native Linux UA
- new "Architecture" section (api, RabbitMQ, worker, Redis) via the optional
  engine block; base engine styles moved into landing.css
- features: window between tasks, inner_text; limits: open window takes a slot

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

assets/css/landing.css

@@ -118,3 +118,23 @@ h3 { font-size: 1.05rem; font-weight: 600; }
 .footer { padding: 2rem 0; border-top: 1px solid var(--border); color: var(--muted); font-size: .9rem; }
 .footer__row { display: flex; flex-wrap: wrap; justify-content: space-between; gap: .5rem 2rem; }
 .footer p { margin: 0; }
+
+/* under the hood (optional "engine" section; themes may restyle it) */
+.engine__kicker {
+  margin: 0 0 .5rem; color: var(--accent);
+  font: 600 .8rem/1 'JetBrains Mono', monospace; text-transform: uppercase; letter-spacing: .12em;
+}
+.engine__list {
+  list-style: none; margin: 2rem 0 0; padding: 0;
+  display: grid; grid-template-columns: repeat(auto-fit, minmax(19rem, 1fr)); gap: .75rem 2rem;
+}
+.engine__list li { position: relative; padding-left: 1.4rem; color: var(--muted); }
+.engine__list li::before { content: "›"; position: absolute; left: 0; color: var(--accent); font-weight: 800; }
+.engine__list strong { color: var(--text); }
+.engine__callout {
+  margin-top: 2.5rem; padding: 1.25rem 1.5rem;
+  border: 1px solid rgba(124, 140, 255, .35); border-radius: var(--radius);
+  background: rgba(124, 140, 255, .06);
+}
+.engine__callout h3 { color: var(--accent); font-size: 1.1rem; }
+.engine__callout p { margin: .5rem 0 0; color: var(--muted); }