Add load_tester and ThreadPipeline.h to docs

design.md

@@ -57,6 +57,9 @@ ninja test    # or ctest
 **Debug tools:**
 - `./debug_arena` - Analyze arena allocator behavior
 
+**Load Testing:**
+- `./load_tester` - A tool to generate load against the server for performance and stability analysis.
+
 ### Dependencies
 
 **System requirements:**
@@ -114,6 +117,15 @@ Ultra-fast memory allocator optimized for request/response patterns:
 - **Streaming data processing** with partial message handling
 - **Connection lifecycle hooks** for initialization and cleanup
 
+#### **Thread Pipeline** (`src/ThreadPipeline.h`)
+
+A high-performance, multi-stage, lock-free pipeline for inter-thread communication.
+
+- **Lock-Free Design**: Uses a shared ring buffer with atomic counters for coordination, avoiding locks for maximum throughput.
+- **Multi-Stage Processing**: Allows items (like connections or data packets) to flow through a series of processing stages (e.g., from I/O threads to worker threads).
+- **Batching Support**: Enables efficient batch processing of items to reduce overhead.
+- **RAII Guards**: Utilizes RAII (`StageGuard`, `ProducerGuard`) to ensure thread-safe publishing and consumption of items in the pipeline, even in the presence of exceptions.
+
 #### **Parsing Layer**
 
 **JSON Commit Request Parser** (`src/json_commit_request_parser.{hpp,cpp}`):