Fix CPX UART flow-control handling

[ataffanel]

Summary

Fix CPX UART flow-control handling in UARTTransport.

The previous implementation used one Lock both to serialize outgoing writes and to represent incoming UART CTS (0xFF 0x00). This could crash with:

RuntimeError: release unlocked lock

when duplicate or unsolicited CTS frames arrived while no write was pending.

This PR replaces that lock-as-CTS mechanism with explicit flow-control state:

• Use threading.Event to track peer CTS
• Use a separate TX lock to serialize outgoing packets
• Use a raw serial write lock to avoid interleaving data frames and CTS replies
• Treat duplicate/unsolicited 0xFF 0x00 frames as harmless CTS events
• Discard CRC-bad frames and continue reading until a valid CPX packet arrives
• Add fake-serial unit tests for CTS handling, CRC recovery, and CTS-gated writes

Fixes #599.

Testing

• python -m unittest test.cpx.test_uart_transport
• python -m unittest discover test