- Add compute_ipv4_subnets() and expand_volatile_subnets_v4() to functions
- Call expand_volatile_subnets_v4 for IPv4 volatile domains in route-update.sh
- Fix shell condition precedence (||/&&) for both IPv4 and IPv6 volatile detection
- Support multiple EXTRA_DNS servers (8.8.8.8, 1.1.1.1, 9.9.9.9)
- Add is_pingonly_gw() for ikev2.fr/ikev2.gr in route-health.sh

Fixes Codex 403: CDN edge IPs (8.47.69.x, 8.6.112.x) were missing from ai table
because DNS returns different IPs than what route-update resolves via dig.
expand_volatile_subnets_v4 accumulates IPs from multiple resolvers and computes
covering /24 subnets automatically.