docs: add latency benchmark vs LiteLLM/Portkey

- bench/ reproducible harness: instant mock backend + Apache Bench (ab -k -r)
- measures gateway overhead in passthrough mode (apples-to-apples)
- BENCHMARKS.md: methodology, caveats, results (cortiq ~57k rps p99 1ms vs
  Portkey ~5.8k p99 9ms vs LiteLLM 4-workers ~1.2k p99 59ms)
- README (EN+RU): Performance section + highlight

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

bench/mock.js

@@ -0,0 +1,40 @@
+// Fast, dependency-free mock upstream for gateway benchmarks.
+// Responds instantly with a fixed OpenAI-compatible chat completion so the
+// measured latency reflects GATEWAY overhead, not model time.
+//
+//   node bench/mock.js [port]   # default 8100
+const http = require("http");
+const PORT = parseInt(process.argv[2] || "8100", 10);
+
+const COMPLETION = JSON.stringify({
+  id: "chatcmpl-bench",
+  object: "chat.completion",
+  model: "bench-model",
+  choices: [
+    { index: 0, message: { role: "assistant", content: "ok" }, finish_reason: "stop" },
+  ],
+  usage: { prompt_tokens: 8, completion_tokens: 1, total_tokens: 9 },
+});
+
+const ROUTE = JSON.stringify({
+  request_id: "bench",
+  decision: { task_label: "general", complexity: { score: 0.2, tier: "low" } },
+});
+
+const server = http.createServer((req, res) => {
+  if (req.method === "GET") {
+    res.writeHead(200, { "content-type": "text/plain" });
+    res.end("ok");
+    return;
+  }
+  // drain the request body, then reply
+  req.on("data", () => {});
+  req.on("end", () => {
+    const body = req.url.includes("/route") ? ROUTE : COMPLETION;
+    res.writeHead(200, { "content-type": "application/json" });
+    res.end(body);
+  });
+});
+
+server.keepAliveTimeout = 60000;
+server.listen(PORT, "127.0.0.1", () => console.log(`mock upstream on :${PORT}`));