Add batching handler and streaming handler examples

design.md

@@ -126,6 +126,7 @@ Key features:
 - Private networking details accessible only to Server via friend relationship
 - Public handler interface: appendMessage(), closeAfterSend(), getArena(), getId()
 - Thread-safe ownership transfer with Server::releaseBackToServer()
+- **Protocol-specific data**: `user_data` `void*` for custom handler data.
 
 #### 9. **ConnectionHandler Interface** (`src/connection_handler.hpp`)
 - **Abstract protocol interface** decoupling networking from application logic
@@ -136,6 +137,7 @@ Key features:
 Key features:
 - on_data_arrived()/on_write_progress() with unique_ptr<Connection>& for ownership transfer
 - on_connection_established/closed() hooks for protocol state management
+- on_post_batch(): Hook for batch processing of connections after I/O
 - Zero-copy data processing with arena allocator integration
 - Thread-safe ownership transfer via Server::releaseBackToServer()
 
@@ -368,7 +370,63 @@ public:
 };
 ```
 
-TODO add example of streaming data
+#### Batching Handler with User Data
+```cpp
+class BatchingHandler : public ConnectionHandler {
+public:
+  void on_connection_established(Connection &conn) override {
+    // Allocate some protocol-specific data and attach it to the connection
+    conn.user_data = new MyProtocolData();
+  }
+
+  void on_connection_closed(Connection &conn) override {
+    // Free the protocol-specific data
+    delete static_cast<MyProtocolData*>(conn.user_data);
+  }
+
+  void on_data_arrived(std::string_view data,
+                       std::unique_ptr<Connection> &conn_ptr) override {
+    // Process data and maybe store some results in the user_data
+    auto* proto_data = static_cast<MyProtocolData*>(conn_ptr->user_data);
+    proto_data->process(data);
+  }
+
+  void on_post_batch(std::span<std::unique_ptr<Connection>> batch) override {
+    // Process a batch of connections
+    for (auto& conn_ptr : batch) {
+      if (conn_ptr) {
+        auto* proto_data = static_cast<MyProtocolData*>(conn_ptr->user_data);
+        if (proto_data->is_ready()) {
+          // This connection is ready for the next stage, move it to the pipeline
+          pipeline_.push(std::move(conn_ptr));
+        }
+      }
+    }
+  }
+
+private:
+  MyProcessingPipeline pipeline_;
+};
+```
+
+#### Streaming "yes" Handler
+```cpp
+class YesHandler : public ConnectionHandler {
+public:
+  void on_connection_established(Connection &conn) override {
+    // Start writing immediately
+    on_write_progress(conn);
+  }
+
+  void on_write_progress(std::unique_ptr<Connection> &conn_ptr) override {
+    // Write "y\n" repeatedly
+    if (conn_ptr->outgoingBytesQueued() == 0)
+        conn_ptr->appendMessage("y\n");
+    }
+  }
+};
+```
+
 
 ### Arena-Based String Handling
 ```cpp