Function validate_write_registers 

pub fn validate_write_registers(
    address: u16,
    quantity: u16,
    byte_count: u8,
) -> Result<(), ExceptionCode>

Validate a write multiple registers (FC 10) request.

Per spec §6.12 Figure 22 state diagram.

Errors

• IllegalDataValue if quantity is 0 or exceeds 123, or byte count doesn’t match
• IllegalDataAddress if address + quantity > 0xFFFF