fix(network): wire default route + bridge DNS for isolated containers

When isolateContainers=true, claude (and all egress) broke in every
container: agents came up with an IP but no way off the bridge subnet.

Two container-side gaps, both confirmed against nixpkgs
nixos-containers.nix:

1. No default route. hive-priv wrote HOST_ADDRESS= empty in the nspawn
   conf. nixos-container's container-side setup only installs
   `ip route add default via $HOST_ADDRESS` when HOST_ADDRESS is
   non-empty, so the container had an address but no gateway -> nothing
   off-subnet (incl. api.anthropic.com) was reachable. Fix: write
   HOST_ADDRESS=<bridge-ip>. In bridge mode the host-side address/route
   setup is skipped, so this only affects the container's default route.

2. No usable resolver. nixos-container copies the host's /etc/resolv.conf
   into the container at every start; the host resolver (e.g. 127.0.0.53)
   is unreachable from a private netns and isn't authoritative for the
   hive's own zones. Fix: hive-priv drops a marker carrying the gateway
   IP only when isolated, and a new harness-base oneshot
   (hyperhive-isolated-dns) rewrites resolv.conf to point at the bridge
   dnsmasq. Inert in shared-netns mode (no marker), so the shared
   container toplevel does the right thing in both modes.

The gateway IP is the address part of HIVE_NETWORK_SUBNET (the bridge IP
verbatim, honouring a non-.1 operator override), via a new validated
bridge_gateway_ip() helper with unit tests.

Unblocks defaulting isolation on.

hive-sh4re/src/priv_proto.rs

@@ -105,6 +105,15 @@ pub struct NetworkIsolation {
     pub agent_ip: String,
     /// Host bridge interface name (e.g. `hive0`).
     pub bridge: String,
+    /// Bridge gateway IP (the host-side bridge address, e.g. `10.42.0.1`).
+    /// Written as `HOST_ADDRESS=` in the nspawn conf so nixos-container's
+    /// container-side setup installs a default route (`default via <gw>`):
+    /// without it the container comes up with an address but no route off
+    /// the bridge subnet — no internet, no `api.anthropic.com`. The same IP
+    /// runs the hive dnsmasq resolver, so it's also written into the
+    /// container's `/etc/resolv.conf` (see the isolated-DNS oneshot in
+    /// `harness-base.nix`, gated on the marker hive-priv drops).
+    pub gateway_ip: String,
 }
 
 /// A request to the privileged helper.