Flush relayed frames once per read batch

The PLAY relay called writeAndFlush for every decoded frame. With
TCP_NODELAY, a socket read holding many small packets (a typical server
tick) cost one flush, write syscall and TCP segment per frame. Write each
frame and flush the peer on channelReadComplete. A per-batch event-loop
task flushes frames forwarded outside a socket read, such as delayed tab
completion. Paused-frame replay keeps writeAndFlush.

ProxySessionBenchmark gains --burst plus proxy I/O CPU and TCP segment
counters. On a 4 vCPU Linux container with 32 x 33-byte frames per write,
TCP segments/frame fell from 1.379 to 0.117 and proxy I/O CPU/frame from
4079 to 1827 ns (medians of six paired phases). The existing 1 KiB window
1 and 16 workloads showed no regression.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01KcCN2hYyqmU22X2Xfg1nwe

docs/architecture.md

@@ -28,7 +28,7 @@ flowchart LR
     F --> G[PLAY / Forge 握手与转发]
 ```
 
-普通转发沿用协议 5 帧边界，不重新编码普通帧。前后端通道绑定同一 Netty EventLoop，会话控制状态在该循环串行变更。Keep Alive ID 由代理转换并跟踪未完成请求；必要的玩家实体 ID 数据包在转服后改写。通道不可写时通过暂停对端读取施加背压。实现见 [`Session`](../proxy-core/src/main/java/dev/moonbridge/core/session/Session.java)、[`RawRelay`](../proxy-core/src/main/java/dev/moonbridge/core/relay/RawRelay.java) 与 [`KeepAliveBridge`](../proxy-core/src/main/java/dev/moonbridge/core/session/KeepAliveBridge.java)。
+普通转发沿用协议 5 帧边界，不重新编码普通帧；同一次 socket 读取解码出的帧逐个写入对端，在读取完成时合并为一次 flush，读取之外转发的帧由每批一次的事件循环任务补发 flush。前后端通道绑定同一 Netty EventLoop，会话控制状态在该循环串行变更。Keep Alive ID 由代理转换并跟踪未完成请求；必要的玩家实体 ID 数据包在转服后改写。通道不可写时通过暂停对端读取施加背压。实现见 [`Session`](../proxy-core/src/main/java/dev/moonbridge/core/session/Session.java)、[`RawRelay`](../proxy-core/src/main/java/dev/moonbridge/core/relay/RawRelay.java) 与 [`KeepAliveBridge`](../proxy-core/src/main/java/dev/moonbridge/core/session/KeepAliveBridge.java)。
 
 ## 后端目录与发现所有权