headscale: move the tailnet to orbit.sol, route all DNS through pihole

Three connected changes, all triggered by the same outage.

base_domain leaves mgaction.town. That zone has a wildcard A+AAAA pointing
at neptun, and DNS wildcards match multi-label names, so
jupiter.hosts.mgaction.town resolved publicly to NEPTUN and Caddy proxied
to itself -- a silent loop rather than a lookup failure. Nesting the
tailnet inside the LAN domain as orbit.sol keeps the theme and resolves
unambiguously, since tailscale matches routes by longest suffix.

override_local_dns = true with pihole as the only global nameserver, so
roaming devices get ad blocking and .sol names off-LAN. With it false,
globalResolvers land in the netmap's FallbackResolvers, which a phone
with carrier DNS never consults. No public fallback is listed on purpose:
tailscale treats the list as a set, so a second entry would let queries
slip past the filter whenever mercury is slow. The cost is that mercury
is now a single point of failure for tailnet DNS.

neptun and mercury opt out individually. mercury would otherwise resolve
through itself. neptun must not depend on a Pi behind a domestic line to
renew the certificates for the control server every other node needs --
and it is circular besides, since tailscaled has to resolve
vpn.mgaction.town to connect at all. Instead neptun runs a dnsmasq stub
forwarding just orbit.sol to MagicDNS on 100.100.100.100, which tailscaled
answers whenever it is running regardless of --accept-dns. That resolves
jupiter live, so the hardcoded /etc/hosts pin is gone.

Also sets dns.nameservers.split explicitly: nixpkgs renders its own
dns.split option one level too high, but headscale reads
dns.nameservers.split (hscontrol/types/config.go:722) and so does
headplane, whose DNS page dies on the missing key with "Cannot convert
undefined or null to object". The module's option is dead as written.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

common.nix

@@ -47,7 +47,7 @@
     options = "--delete-older-than 30d";
   };
 
-  environment.systemPackages = with pkgs; [ vim git htop tmux curl ];
+  environment.systemPackages = with pkgs; [ vim git htop tmux curl wget ];
 
   # ---- Locale / firewall base ----
   time.timeZone = "Europe/Berlin";