feat(build-stack): mDNS preflight warnings (host avahi, dbus, interface count)

When ENABLE_MDNS=true, warn if BASE_DOMAIN isn't .local, host avahi-daemon
isn't running, the D-Bus socket is missing, or the host has many interfaces
(avahi self-collision risk → suggests allow-interfaces).

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

tooling/build-stack.sh

@@ -116,6 +116,13 @@ up() {
       *.local) ;;
       *) warn "ENABLE_MDNS=true but BASE_DOMAIN=$BASE_DOMAIN is not *.local — mDNS won't resolve it" ;;
     esac
+    pgrep -x avahi-daemon >/dev/null 2>&1 \
+      || warn "host avahi-daemon not detected — the avahi container publishes through it; start it (e.g. 'systemctl start avahi-daemon')"
+    [ -S /run/dbus/system_bus_socket ] \
+      || warn "host D-Bus socket /run/dbus/system_bus_socket missing — avahi publish will fail"
+    local ifc; ifc=$(ip -o link show 2>/dev/null | wc -l)
+    [ "${ifc:-0}" -gt 6 ] \
+      && warn "host has ${ifc} network interfaces — set 'allow-interfaces=<lan-nic>' in /etc/avahi/avahi-daemon.conf, else avahi self-collides (Local name collision)"
   fi
   log "PHASE up (mode=$mode${ENABLE_MDNS:+, mdns=$ENABLE_MDNS})"
   have_script render-config.sh