ateomnet: worker pod netns enables IPv4 forwarding only, dropping actor IPv6 traffic

Sub-task of #246, part of bowei's [step 3](https://github.com/agent-substrate/substrate/issues/246#issuecomment-4771177881).

`EnableIPv4Forwarding` writes `/proc/sys/net/ipv4/ip_forward` and has no IPv6 counterpart (`internal/ateomnet/net.go:206`). Actor packets enter the worker pod on a veth and are forwarded to `eth0`, so with `net.ipv6.conf.all.forwarding` left at 0 every IPv6 packet an actor sends is dropped by `ip6_forward()` — including the actor's own DNS queries.

On an IPv6-only cluster this is a hard failure: `TestActorEgress` returns 504, reproduced 4/4. The netns setup also has no IPv6 veth addressing and no v6 nftables rules, per the existing TODO at `net.go:243`.