Add configurable transmit delay via set_transmit_delay()

[brocci]

Summary

While the v1.7.x library now supports INIT messages and allow for an init handler, it lacks a way to set the transmit delay.

NMRA CMRInet (LCS-9.10.1) specifies a per-character transmission delay derived from the dH/dL bytes in the INIT message. Each unit represents 10 microseconds. Modern hosts set dH/dL to zero; non-zero values are for legacy compatibility with older Host computers.

The library hardcodes delayMicroseconds(50) in transmit() as an RS-485 turnaround delay, regardless of the commanded dH/dL value.

This PR adds a set_transmit_delay() method and a compile-time default via #define TRANSMIT_DELAY_US, allowing sketches to configure the turnaround delay. Sketches can parse dH/dL in an init handler and apply the delay automatically for legacy host compatibility.

Changes

Compile-time default (CMRI.h)

#ifndef TRANSMIT_DELAY_US
// RS-485 turnaround delay before transmitting (microseconds)
// Set to 0 if DE timing is handled externally
#define TRANSMIT_DELAY_US 50
#endif

Runtime override (CMRI.h, CMRI.cpp)

void set_transmit_delay(unsigned int delay_us);

New public method stores the delay in _transmit_delay_us. Applied in transmit() before sending the GET frame:

void CMRI::transmit()
{
    delayMicroseconds(_transmit_delay_us);
    _serial.write(255);
    // ...
}

INIT handler example (examples/init_handler_with_delay/)

Demonstrates parsing dH/dL from the INIT payload and applying the delay:

void on_init(const uint8_t *data, int len)
{
    if (len >= 3)
    {
        unsigned int delay_us = (data[1] * 256 + data[2]) * 10;
        cmri.set_transmit_delay(delay_us);
    }
}

Unit test

Added test_set_transmit_delay to verify the setter compiles and transmit() still produces a valid frame.

Backward compatibility

• Default TRANSMIT_DELAY_US is 50, matching previous hardcoded behavior
• set_transmit_delay() is optional -- existing sketches work unchanged
• Setting delay to 0 disables the wait (for full-duplex or external DE control)