Routing apps (a rootless VPN exit) need packet forwarding in their own
network namespace, but /proc/sys is read-only inside a rootless
container, so it can only be set at create time. Add `app.sysctls`,
allow-listed to net.ipv4.ip_forward / net.ipv6.conf.all.forwarding with
values "0"/"1", and rejected under host networking where it would change
the host. Rendered on all three create paths: podman CLI --sysctl, the
libpod spec `sysctl` map, and Quadlet `Sysctl=`. Absent by default and
not serialized when empty, so existing manifests and units are unchanged.
Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>