Add outbound writer thread with batching and non-blocking send; extract Connection write/flush

[ArtifactForms]

Motivation

• Reduce contention and avoid blocking the read thread by moving packet writes off the read path and batching outbound packets.
• Make low-level packet write and flush operations overridable/extractable for reuse and testing.

Description

• Extracted writePacket(Packet) and flushOutput() protected methods from Connection.send and replaced the inlined write/flush calls with delegations to those methods in Connection.
• Reworked ServerConnection to use a non-blocking outbound queue (LinkedBlockingQueue<Packet>) and a dedicated writer thread that polls the queue, batches up to MAX_OUTBOUND_BATCH_SIZE packets, and writes them via writeBatch.
• Implemented a writeLoop that polls with a timeout (WRITER_POLL_TIMEOUT_MS) to allow graceful shutdown and drains available packets into batches for efficient network writes.
• Overrode send(Packet) in ServerConnection to enqueue outbound packets instead of performing synchronous I/O, and updated close() to interrupt the writer thread and unregister the connection from the PlayerManager.

Testing

• Ran the project's automated unit test suite and server integration tests against the modified networking code and observed all tests passing.
• Performed basic concurrency and shutdown validation by exercising client connect/send/close flows in integration tests which completed without deadlocks or uncaught exceptions.

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Codex Task