Add Connection::reset

There was an issue where the connection deque was referring to old arena memory

design.md

@@ -121,7 +121,6 @@ Key features:
 
 Key features:
 - Arena allocator per connection for efficient memory management
-- **Request/Response arena lifecycle**: Arena resets after each complete request/response cycle
 - Weak reference to server for safe cleanup after server destruction
 - Private networking details accessible only to Server via friend relationship
 - Public handler interface: appendMessage(), closeAfterSend(), getArena(), getId()
@@ -180,9 +179,10 @@ Arena Memory Lifecycle:
 2. **Response Generation**: Handler uses arena for temporary response construction (headers, JSON, etc.)
 3. **Response Queuing**: Handler calls `conn->appendMessage()` which copies data to arena-backed message queue
 4. **Response Writing**: Server writes all queued messages to socket via `writeBytes()`
-5. **Arena Reset**: After successful write completion, arena resets to reclaim all memory from the request/response cycle
 
-This design assumes request/response pairs (HTTP-like protocols) but works for any protocol where there's a clear completion point for memory reclamation.
+The user is responsible for calling `conn->reset()` periodically to reclaim the
+arena memory. A good time to do this for a request/response protocol would be
+after all outgoing bytes have been written.
 
 ### API Design
 

src/connection.cpp

@@ -118,10 +118,6 @@ bool Connection::writeBytes() {
   }
   assert(messages_.empty());
 
-  // Reset arena after completing request/response cycle
-  // This reclaims memory from request parsing and response generation
-  arena_.reset();
-
   return false;
 }
 

src/connection.hpp

@@ -52,6 +52,13 @@ struct Connection {
     return result;
   }
   void *user_data = nullptr;
+  void reset() {
+    assert(messages_.empty());
+    arena_.reset();
+    messages_ =
+        std::deque<std::string_view, ArenaStlAllocator<std::string_view>>{
+            ArenaStlAllocator<std::string_view>{&arena_}};
+  }
 
   // Note: To release connection back to server, use
   // Server::releaseBackToServer(std::move(connection_ptr))