feat(tooling): mdns-host-setup.sh — pin host avahi to the LAN NIC

Auto-detects the LAN interface from HOST_LAN_IP, sets allow-interfaces in
/etc/avahi/avahi-daemon.conf (idempotent, backs up), restarts avahi. Self-sudos.
Fixes the multi-bridge 'Local name collision' that blocks the mDNS path.

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

tooling/mdns-host-setup.sh

@@ -0,0 +1,48 @@
+#!/usr/bin/env bash
+# mdns-host-setup.sh — pin the HOST avahi-daemon to the LAN interface so it
+# stops self-colliding across docker bridges ("Local name collision"), which
+# is what blocks the mDNS (.local) path on a host with many interfaces.
+#
+# Detects the LAN NIC from HOST_LAN_IP (.env), sets allow-interfaces=<nic> in
+# /etc/avahi/avahi-daemon.conf (idempotent, backs up first), restarts avahi.
+# Re-execs itself with sudo if not root. Run: tooling/mdns-host-setup.sh
+set -euo pipefail
+cd "$(dirname "$0")/.."
+
+[ -f .env ] && { set -a; . ./.env; set +a; }
+: "${HOST_LAN_IP:?HOST_LAN_IP unset (set in .env)}"
+
+nic="$(ip -o -4 addr show | awk -v ip="$HOST_LAN_IP" 'index($0, " "ip"/"){print $2; exit}')"
+[ -n "$nic" ] || { echo "ERROR: no interface found with IP $HOST_LAN_IP" >&2; exit 1; }
+echo "LAN interface for ${HOST_LAN_IP}: ${nic}"
+
+conf=/etc/avahi/avahi-daemon.conf
+[ -f "$conf" ] || { echo "ERROR: $conf not found — is avahi-daemon installed?" >&2; exit 1; }
+
+# Need root to edit the conf + restart the service.
+if [ "$(id -u)" -ne 0 ]; then
+  echo "escalating with sudo…"
+  exec sudo "$0" "$@"
+fi
+
+# Already pinned to this NIC? nothing to do.
+if grep -qE "^allow-interfaces=${nic}([,[:space:]]|$)" "$conf"; then
+  echo "already set: allow-interfaces=${nic}"
+else
+  cp -a "$conf" "${conf}.bak.$(date +%s)"
+  if grep -qE '^allow-interfaces=' "$conf"; then
+    sed -i -E "s/^allow-interfaces=.*/allow-interfaces=${nic}/" "$conf"
+  elif grep -qE '^#\s*allow-interfaces=' "$conf"; then
+    sed -i -E "s/^#\s*allow-interfaces=.*/allow-interfaces=${nic}/" "$conf"
+  else
+    # insert right after the [server] section header
+    sed -i -E "/^\[server\]/a allow-interfaces=${nic}" "$conf"
+  fi
+  echo "set allow-interfaces=${nic} (backup: ${conf}.bak.*)"
+fi
+
+echo "restarting avahi-daemon…"
+systemctl restart avahi-daemon
+sleep 1
+systemctl is-active --quiet avahi-daemon && echo "avahi-daemon active" || { echo "ERROR: avahi-daemon not active" >&2; exit 1; }
+grep -E '^allow-interfaces=' "$conf"