Add fault injection and sender-side metrics for reliability testing

[only-komal]

This PR adds configurable packet loss fault injection to the CTP sender,
allowing simulation of unreliable links.

It also introduces sender-side metrics to quantify:

• total frames sent
• retransmissions triggered by packet loss

Usage:
./sender
Drops approximately 1 in N frames and reports transmission behavior.

This helps evaluate protocol reliability under controlled fault conditions.