Remove release_back_to_server

design.md

@@ -243,19 +243,19 @@ CommitRequest {
 
 #### Connection Ownership Lifecycle
 
-1. **Creation**: Accept threads create connections, transfer to epoll as raw pointers
-1. **Processing**: Network threads claim ownership by wrapping in unique_ptr
-1. **Handler Transfer**: Handlers can take ownership for async processing via unique_ptr.release()
-1. **Return Path**: Handlers use Server::release_back_to_server() to return connections
-1. **Safety**: All transfers use weak_ptr to server for safe cleanup
-1. **Cleanup**: RAII ensures proper resource cleanup in all scenarios
+1. **Creation**: Server creates connections and stores them in registry
+1. **Processing**: I/O threads access connections via registry lookup
+1. **Handler Access**: Handlers receive Connection& references, server retains ownership
+1. **Async Processing**: Handlers use WeakRef<Connection> for safe async access
+1. **Safety**: Connection mutex synchronizes concurrent access between threads
+1. **Cleanup**: RAII ensures proper resource cleanup when connections are destroyed
 
 #### Arena Memory Lifecycle
 
-1. **Request Processing**: Handler uses `conn->get_arena()` to allocate memory for parsing request data
-1. **Response Generation**: Handler uses arena for temporary response construction (headers, JSON, etc.)
-1. **Response Queuing**: Handler calls `conn->append_message()` which copies data to arena-backed message queue
-1. **Response Writing**: Server writes all queued messages to socket via `writeBytes()`
+1. **Request Processing**: Handler creates request-scoped arena for parsing request data
+1. **Response Generation**: Handler uses same arena for response construction (headers, JSON, etc.)
+1. **Response Queuing**: Handler calls `conn->append_message()` passing span + arena ownership
+1. **Response Writing**: I/O thread writes messages to socket, arena freed after completion
 
 > **Note**: Call `conn->reset()` periodically to reclaim arena memory. Best practice is after all outgoing bytes have been written.