hive-gateway: note common-case dnsmasq address redundancy (argus 🟡 on #845)

forge/matrix subdomain address rules are redundant when those are
sub-domains of hyperhive.domain (dnsmasq's /<domain>/ matches
sub-domains). Keep explicit for the cross-domain case (e.g.
forge.domain = "git.example.com") + add a comment explaining
the trade-off.

nix/modules/hive-gateway.nix

@@ -587,6 +587,17 @@ in
               # (where nginx is reachable from container netns once
               # #14 lands; today it's the host loopback alias and
               # works in either shape).
+              #
+              # The forge / matrix entries are redundant in the
+              # common case where `forge.domain` /
+              # `matrix.gatewayHost` are sub-domains of
+              # `hyperhive.domain` — dnsmasq's `/<domain>/` rule
+              # already matches sub-domains (argus 🟡 on #845).
+              # Kept explicit because operators can override either
+              # to a cross-domain hostname (e.g.
+              # `forge.domain = "git.example.com"`); listing them
+              # explicitly keeps that case routed without needing
+              # an extra config block.
               address = [
                 "/${hyperhiveDomain}/${networkCfg.bridgeIp}"
               ]