docs(host.env): GNOME/Mutter example for an Ubuntu desktop host

Prep for a third (Ubuntu) test host: document the Mutter backend env — wayland-0
(not wayland-kde), XDG_CURRENT_DESKTOP=GNOME, PUNKTFUNK_COMPOSITOR=mutter, virtual
source via RecordVirtual, libei input via the RemoteDesktop portal.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

scripts/host.env.example

@@ -24,6 +24,18 @@ PUNKTFUNK_ZEROCOPY=1
 # Mutually exclusive with the above: ATTACH to a gamescope session something ELSE owns (fixed mode):
 #PUNKTFUNK_GAMESCOPE_NODE=auto       # discover + capture a running gamescope (do NOT combine with SESSION)
 
+# --- GNOME / Mutter host (e.g. an Ubuntu desktop) -----------------------------------------
+# Attach to a running GNOME (Wayland) session — its default socket is wayland-0, not wayland-kde.
+# Mutter creates the per-client virtual output via its `RecordVirtual` D-Bus API (a virtual
+# monitor alongside any real one), and input goes through the RemoteDesktop portal (libei). On a
+# real desktop the host runs as the logged-in user; headless GNOME also works (gnome-shell
+# --headless). Needs GNOME ≥ 48 for the zero-copy RecordVirtual path.
+#WAYLAND_DISPLAY=wayland-0
+#XDG_CURRENT_DESKTOP=GNOME
+#PUNKTFUNK_COMPOSITOR=mutter
+#PUNKTFUNK_VIDEO_SOURCE=virtual
+#PUNKTFUNK_INPUT_BACKEND=libei
+
 # Optional overrides (apps.json is the primary mechanism for per-app settings):
 #PUNKTFUNK_COMPOSITOR=kwin          # kwin | mutter | gamescope | wlroots
 #PUNKTFUNK_GAMESCOPE_APP=vkcube     # nested command for ad-hoc bare-gamescope sessions