tmux-ip: prefer DigitalOcean reserved/floating IP when on a droplet

If /sys/class/dmi/id/sys_vendor says DigitalOcean and the droplet has
a reserved_ip or floating_ip assigned, use that over the ipify-based
egress IP. Keeps the ipify/hostname -I fallback intact for other hosts.

Gated on the DMI check so non-DO hosts skip the metadata probe entirely
and the tmux status bar refresh stays fast.

README.md

@@ -37,8 +37,9 @@ On top of stock YADR:
 
 - **`.tmux.conf`** — real file (not the YADR symlink), with internal/public IP
   in the status bar and 80k-line scrollback
-- **`.local/bin/tmux-ip`** — helper used by `.tmux.conf`; prefers internal IP
-  with a public-IP fallback
+- **`.local/bin/tmux-ip`** — helper used by `.tmux.conf`; prefers a
+  DigitalOcean reserved/floating IP if the host is a droplet with one
+  attached, falls back to public egress (ipify) and then first local IP
 - **`.irssi/`** — IRC client config (no credentials committed)
 - **`.fonts/`** — Inconsolata, Menlo-Powerline, mensch-Powerline, monof55/56
 - **`.themes/`** — BlackMATE v2 / v2.2 / "The Blues" GNOME metathemes