Implement PL2303 RTS/CTS flow control and safety cut logging

## Context

TODOs for serial flow control and safety logging.

## TODOs

1. **PL2303SerialToUsbConnector.java:170** — implement RTS/CTS flow control for PL2303
2. **LogItems.java:102** — implement safety cut logging

## References

- `PL2303SerialToUsbConnector.java:170`
- `LogItems.java:102`

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*Created from workspace-wide TODO audit.*