Optimize RX buffering for high-frequency input

[BBBQL2021]

Summary

• add bounded RX buffer with periodic flushing and chunking to reduce WebSocket backpressure under high-frequency traffic
• increase serial read size and protect against buffer overflow to keep receiver responsive
• harden WebSocket broadcast handling to drop broken clients instead of blocking the pipeline

Testing

• python -m compileall app.py

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Codex Task