Add async batched writer to ServerConnection and extract write/flush hooks in Connection

[ArtifactForms]

Motivation

• Reduce contention and risk of packet corruption by moving outgoing writes off the caller thread into a dedicated writer thread.
• Improve throughput by batching outbound packets and minimizing per-packet flushes.
• Provide handover points for subclasses to customize packet serialization and flushing via writePacket and flushOutput.

Description

• Extracted writePacket(Packet) and flushOutput() from Connection.send() and delegated the original send implementation to call those helpers.
• Implemented an asynchronous outbound pipeline in ServerConnection with a LinkedBlockingQueue<Packet> outboundQueue, a daemon writerThread and a writeLoop() that polls and batches packets.
• Overrode ServerConnection.send(Packet) to enqueue packets instead of writing synchronously, and added writeBatch(...) for synchronized batched writes that call writePacket and flushOutput.
• Ensure clean shutdown by interrupting the writer thread in close() and handling write failures by logging and closing the connection.

Testing

• Ran the project's automated test suite with ./gradlew test, which completed successfully.
• Executed automated network integration smoke tests that start the server, establish client connections, exchange packets under load, and verify delivery and clean shutdown; these tests passed.
• Ran the connection-level unit tests exercising send/write batching and graceful close behavior, which passed.

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Codex Task