Add Cookbook S01-S22 (draft)

docs-src/pages/en/cookbook/s16-disconnect.md

@@ -0,0 +1,55 @@
+---
+title: "S16. Detect When the Client Has Disconnected"
+order: 35
+status: "draft"
+---
+
+During a long-running response, the client might close the connection. There's no point continuing to do work no one's waiting for. In cpp-httplib, check `req.is_connection_closed()`.
+
+## Basic usage
+
+```cpp
+svr.Get("/long-task", [](const httplib::Request &req, httplib::Response &res) {
+  for (int i = 0; i < 1000; ++i) {
+    if (req.is_connection_closed()) {
+      std::cout << "client disconnected" << std::endl;
+      return;
+    }
+
+    do_heavy_work(i);
+  }
+
+  res.set_content("done", "text/plain");
+});
+```
+
+`is_connection_closed` is a `std::function<bool()>`, so call it with `()`. It returns `true` when the client is gone.
+
+## With a streaming response
+
+The same check works inside `set_chunked_content_provider()`. Capture the request by reference.
+
+```cpp
+svr.Get("/events", [](const httplib::Request &req, httplib::Response &res) {
+  res.set_chunked_content_provider(
+    "text/event-stream",
+    [&req](size_t offset, httplib::DataSink &sink) {
+      if (req.is_connection_closed()) {
+        sink.done();
+        return true;
+      }
+
+      auto event = generate_next_event();
+      sink.write(event.data(), event.size());
+      return true;
+    });
+});
+```
+
+When you detect a disconnect, call `sink.done()` to stop the provider from being called again.
+
+## How often should you check?
+
+The call itself is cheap, but calling it in a tight inner loop doesn't add much value. Check at **boundaries where interrupting is safe** — after producing a chunk, after a database query, etc.
+
+> **Warning:** `is_connection_closed()` is not guaranteed to reflect reality instantly. Because of how TCP works, sometimes you only notice the disconnect when you try to send. Don't expect pixel-perfect real-time detection — think of it as "we'll notice eventually."