Function validate_write_single_coil 

pub fn validate_write_single_coil(coil_value: u16) -> Result<(), ExceptionCode>

Validate a write single coil (FC 05) request.

Per spec §6.5 Figure 15 state diagram — value must be 0x0000 or 0xFF00.

Errors

• IllegalDataValue if coil_value is not 0x0000 or 0xFF00