Add arrow back to io threads in request flow diagram

threading_performance_report.md

@@ -39,13 +39,13 @@ WeaselDB achieved 1.3M requests/second throughput using a two-stage ThreadPipeli
 ### Request Flow
 ```
 I/O Threads (8) → HttpHandler::on_batch_complete() → ThreadPipeline
-                                                          ↓
+    ↑                                                        ↓
-                                                   Stage 0: Noop thread
+    |                                                 Stage 0: Noop thread
-                                                   (396ns serial CPU per request)
+    |                                                 (396ns serial CPU per request)
-                                                          ↓
+    |                                                        ↓
-                                                   Stage 1: Connection return
+    |                                                 Stage 1: Connection return
-                                                          ↓
+    |                                                        ↓
-                                             Server::release_back_to_server()
+    └─────────────────────── Server::release_back_to_server()
 ```
 
 ### Pipeline Configuration