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-net

@@ -0,0 +1,17 @@
+#!/bin/bash
+#
+# Emit the tmux status-bar network segment.
+#   No VPN:  "<local_ip>"
+#   VPN up:  "<local_ip> (local) / <vpn_ip> (vpn)"
+#
+
+set -u
+
+local_ip=$("$HOME/.local/bin/tmux-ip" 2>/dev/null || true)
+vpn_ip=$("$HOME/.local/bin/tmux-vpn-ip" 2>/dev/null || true)
+
+if [ -n "${vpn_ip:-}" ]; then
+  printf '\xe2\x8c\x82 %s / \xe2\x87\xa1 %s\n' "${local_ip:-}" "$vpn_ip"
+else
+  printf '%s\n' "${local_ip:-}"
+fi