perf(net): borrow inbound packet storage

Decode headers, optional fields, fragments, and single-fragment messages directly over pooled datagrams. Copy only fragment state that crosses a datagram lifetime, preserve synchronous dispatch and ACK ordering, and lock the path to the owned decoder with differential and zero-allocation tests.

src/AcDream.Core.Net/Messages/GameEventDispatcher.cs

@@ -17,7 +17,10 @@ namespace AcDream.Core.Net.Messages;
 /// Handlers are invoked synchronously on the thread that called
 /// <see cref="Dispatch"/> — normally the render thread since
 /// <see cref="WorldSession"/>'s decode path runs there in the current
-/// architecture. Handlers must not block.
+/// architecture. Handlers must not block or retain the envelope/payload:
+/// production payload memory borrows a pooled inbound datagram and expires
+/// when the synchronous dispatch call returns. Handlers must parse or copy
+/// any state they need to keep.
 /// </para>
 /// </summary>
 public sealed class GameEventDispatcher
@@ -86,7 +89,8 @@ public sealed class GameEventDispatcher
     /// <summary>
     /// Route an envelope to its handler, or log as unhandled. Exceptions
     /// inside handlers are swallowed to keep the decode loop alive — a
-    /// malformed event from the server should not crash the client.
+    /// malformed event from the server should not crash the client. The
+    /// handler must not retain <paramref name="envelope"/> or its payload.
     /// </summary>
     public void Dispatch(GameEventEnvelope envelope)
     {