wait_for_ip now tries --source lease first then --source arp; both produce
identical output handled by parse_lease_ip. Removes the suid leaseshelper
dependency introduced and backed out in Task 3. New unit test confirms
parse_lease_ip works on --source arp output format.
Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>