Traffic generator

[pgigis]

Summary

Tweaks the traffic generator preflight checks to be a bit clearer and less strict around route validation for spoofed traffic.
Also makes command failure handling and reporting a bit cleaner so the preflight output is easier to interpret.

Testing

• cmake -S . -B build && cmake --build build
• Unit tests (ctest --test-dir build) if available
• Other (describe): Does not change anything in the system.

Notes

• No flag or config changes.
• Small behavior change in preflight reporting: spoofed-source route checks are treated more as guidance than hard failure signals.