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

[Copilot]

Pull request overview

This PR fixes CPX UART flow-control handling in UARTTransport by replacing the previous “lock-as-CTS” mechanism (which could trigger RuntimeError: release unlocked lock on duplicate/unsolicited CTS frames) with explicit flow-control state, and adds unit tests to validate the new behavior.

Changes:

• Replace CTS signaling based on a shared Lock with a threading.Event (_tx_ready) and a separate TX serialization lock (_tx_lock).
• Add _serial_write_lock + _write_raw() to prevent interleaved raw serial writes between CTS replies and data frames.
• Update UART read loop to treat unsolicited CTS as harmless and to discard CRC-bad frames until a valid CPX packet is parsed.
• Add fake-serial unit tests covering unsolicited CTS handling, CRC recovery, and CTS-gated writes.

Reviewed changes

Copilot reviewed 3 out of 3 changed files in this pull request and generated no comments.

File	Description
cflib/cpx/transports.py	Reworks UART CTS flow-control and improves read robustness (CRC discard + CTS handling).
test/cpx/test_uart_transport.py	Adds focused unit tests for the updated UART flow-control and CRC recovery behavior.
test/cpx/__init__.py	Marks test.cpx as a package to support test discovery/imports.

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