Add radvd to gateway role to advertise Pi as high-preference IPv6
default router using the stable ULA prefix (fd1e:.../64). With
FritzBox also sending RAs, devices end up with ECMP between Pi and
FritzBox. To solve this, add network_ipv6_gateway (Pi's link-local)
as a static route with metric 100 to all managed hosts — beats RA
metric 425, ensuring all IPv6 default traffic goes through Pi.

Fix IPv6 MASQUERADE in gateway-apply-rules:
- Direct mode: add MASQUERADE on end0 (LAN devices use ULA source
  addresses not known to FritzBox, so Pi must NAT them)
- FORWARD rules: restrict to RELATED,ESTABLISHED only — previously
  the broad ACCEPT rule passed un-NAT'd packets alongside masqueraded
  copies, causing duplicate SYNs, conntrack corruption, and RSTs
- MASQUERADE/clear rules: match by interface not by source subnet
  (devices may use any source address, not just the ULA prefix)
- VPN mode return traffic: explicitly restrict to wg+→end0 direction

Add network_ipv6_gateway var (optional) to network role NM templates
(ethernet, wifi, bridge) — injects a static IPv6 default route at
metric 100 when set. Add rpi5 static route to FritzBox link-local so
Pi keeps IPv6 after FritzBox RA is disabled.

Force SSH to IPv4 for *.local hosts (AddressFamily inet) — prevents
Ansible from hanging on mDNS returning multiple IPv6 addresses.

Update gateway and pihole READMEs with two-step IPv6 setup process.

Co-Authored-By: Claude.ai

Remove static IPv6 support from network role — all hosts use SLAAC
(method=auto). Simplifies NM templates, argument_specs, and resolved.conf.
gateway sysctl accept_ra=2 is now unconditional when gateway_enabled.

Co-authored-by: Claude.ai