tmux status: show VPN IP when tunnel is active

Add tmux-vpn-ip helper (parses ifconfig for utun* on macOS, falls back
to tun/wg/ppp on Linux) and a tmux-net wrapper that conditionally
emits "⌂ <local> / ⇡ <vpn>" when a VPN tunnel is up, or just "<local>"
when it's not. status-right now calls tmux-net.

.local/bin/tmux-vpn-ip

@@ -0,0 +1,24 @@
+#!/bin/bash
+#
+# Print the IPv4 of the first active VPN tunnel interface, if any.
+# Empty output when no VPN is up.
+#
+# macOS: utun*   Linux: tun*, wg*, ppp*
+
+set -u
+
+case "$(uname -s)" in
+  Darwin)
+    ifconfig 2>/dev/null | awk '
+      /^utun[0-9]+:/   { iface=$1; sub(":", "", iface); next }
+      /^[a-z]+[0-9]*:/ { iface="" }
+      iface != "" && $1 == "inet" { print $2; exit }
+    '
+    ;;
+  Linux)
+    if command -v ip >/dev/null 2>&1; then
+      ip -4 -o addr show 2>/dev/null \
+        | awk '$2 ~ /^(tun|wg|ppp)[0-9]+/ { sub("/.*","",$4); print $4; exit }'
+    fi
+    ;;
+esac