Add Cookbook C01-C19 (draft)

docs-src/pages/en/cookbook/c09-chunked-upload.md

@@ -0,0 +1,47 @@
+---
+title: "C09. Send the Body with Chunked Transfer"
+order: 9
+status: "draft"
+---
+
+When you don't know the body size up front — for data generated on the fly or piped from another stream — use `ContentProviderWithoutLength`. The client sends the body with HTTP chunked transfer encoding.
+
+## Basic usage
+
+```cpp
+httplib::Client cli("http://localhost:8080");
+
+auto res = cli.Post("/stream",
+  [&](size_t offset, httplib::DataSink &sink) {
+    std::string chunk = produce_next_chunk();
+    if (chunk.empty()) {
+      sink.done(); // done sending
+      return true;
+    }
+    return sink.write(chunk.data(), chunk.size());
+  },
+  "application/octet-stream");
+```
+
+The lambda's job is just: produce the next chunk and send it with `sink.write()`. When there's no more data, call `sink.done()` and you're finished.
+
+## When the size is known
+
+If you **do** know the total size ahead of time, use the `ContentProvider` overload (taking `size_t offset, size_t length, DataSink &sink`) and pass the total size as well.
+
+```cpp
+size_t total_size = get_total_size();
+
+auto res = cli.Post("/upload", total_size,
+  [&](size_t offset, size_t length, httplib::DataSink &sink) {
+    auto data = read_range(offset, length);
+    return sink.write(data.data(), data.size());
+  },
+  "application/octet-stream");
+```
+
+With a known size, the request carries a Content-Length header — so the server can show progress. Prefer this form when you can.
+
+> **Detail:** `sink.write()` returns a `bool` indicating whether the write succeeded. If it returns `false`, the connection is gone — return `false` from the lambda to stop.
+
+> If you're just sending a file, `make_file_body()` is easier. See C08. POST a File as Raw Binary.