_default_gateway() took whichever 0.0.0.0 line route print printed first,
not the one Windows actually uses (lowest metric). With a VPN adapter
installed (Radmin VPN, Tailscale, ...) its default-ish route can print
before the real one. Verified live: was reporting Radmin VPN's 26.0.0.1
while Ethernet (the interface actually carrying traffic, confirmed via
primary_interface_ip()) has gateway 192.168.3.1; now reports that correctly.
Parsing extracted into pure _parse_windows_default_gateway(), tested against
a captured multi-adapter route print sample.