Remove release_back_to_server

src/arena.hpp

@@ -65,21 +65,19 @@
  * at a time
  *
  * ### Thread Ownership Model:
- * 1. **Network Thread**: Claims connection ownership, accesses arena for I/O
- * buffers
- * 2. **Handler Thread**: Can take ownership via unique_ptr.release(), uses
- * arena for request parsing and response generation
- * 3. **Background Thread**: Can receive ownership for async processing, uses
- * arena for temporary data structures
- * 4. **Return Path**: Connection (and its arena) safely returned via
- *    Server::release_back_to_server()
+ * 1. **I/O Thread**: Server owns connections, processes socket I/O events
+ * 2. **Handler Thread**: Receives Connection& reference, creates request-scoped
+ * arenas for parsing and response generation
+ * 3. **Pipeline Thread**: Can use WeakRef<Connection> for async processing,
+ * creates own arenas for temporary data structures
+ * 4. **Arena Lifecycle**: Request-scoped arenas moved to message queue, freed
+ * after I/O completion without holding connection mutex
  *
  * ### Why This Design is Thread-Safe:
- * - **Exclusive Access**: Only the current owner thread should access the arena
- * - **Transfer Points**: Ownership transfers happen at well-defined
- * synchronization points with proper memory barriers.
- * - **No Shared State**: Each arena is completely isolated - no shared data
- *   between different arena instances
+ * - **Request-Scoped**: Each request gets its own Arena instance for isolation
+ * - **Move Semantics**: Arenas transferred via move, avoiding shared access
+ * - **Deferred Cleanup**: Arena destruction deferred to avoid malloc contention
+ *   while holding connection mutex
  *
  * @warning Do not share Arena instances between threads. Use separate
  * instances per thread or per logical unit of work.