Add Cookbook S01-S22 (draft)

docs-src/pages/en/cookbook/s17-bind-any-port.md

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+---
+title: "S17. Bind to Any Available Port"
+order: 36
+status: "draft"
+---
+
+Standing up a test server often hits port conflicts. With `bind_to_any_port()`, you let the OS pick a free port and then read back which one it gave you.
+
+## Basic usage
+
+```cpp
+httplib::Server svr;
+
+svr.Get("/", [](const auto &req, auto &res) {
+  res.set_content("hello", "text/plain");
+});
+
+int port = svr.bind_to_any_port("0.0.0.0");
+std::cout << "listening on port " << port << std::endl;
+
+svr.listen_after_bind();
+```
+
+`bind_to_any_port()` is equivalent to passing `0` as the port — the OS assigns a free one. The return value is the port actually used.
+
+After that, call `listen_after_bind()` to start accepting. You can't combine bind and listen into a single call here, so you work in two steps.
+
+## Useful in tests
+
+This pattern is great for tests that spin up a server and hit it.
+
+```cpp
+httplib::Server svr;
+svr.Get("/ping", [](const auto &, auto &res) { res.set_content("pong", "text/plain"); });
+
+int port = svr.bind_to_any_port("127.0.0.1");
+std::thread t([&] { svr.listen_after_bind(); });
+
+// run the test while the server is up on another thread
+httplib::Client cli("127.0.0.1", port);
+auto res = cli.Get("/ping");
+assert(res && res->body == "pong");
+
+svr.stop();
+t.join();
+```
+
+Because the port is assigned at runtime, parallel test runs don't collide.
+
+> **Note:** `bind_to_any_port()` returns `-1` on failure (permission errors, no available ports, etc.). Always check the return value.
+
+> To stop the server, see S19. Shut Down Gracefully.