feat: M2 — GNOME/Mutter virtual-display backend (RecordVirtual) + preferred-mode negotiation

Third compositor on the VirtualDisplay seam, via Mutter's direct D-Bus APIs (the
gnome-remote-desktop headless model, no portal grant): RemoteDesktop.CreateSession →
ScreenCast.CreateSession({remote-desktop-session-id}) → Session.RecordVirtual (creates a
virtual monitor) → Start → PipeWireStreamAdded(node_id). A keepalive thread owns the zbus
connection (sessions die with it — RAII teardown); select with LUMEN_COMPOSITOR=mutter or
XDG_CURRENT_DESKTOP=GNOME.

Mutter sizes its virtual monitor FROM the PipeWire format negotiation, so VirtualOutput
gains preferred_mode (w, h, refresh_hz), threaded into the consumer's format pods as the
default size/framerate. KWin/gamescope set it too (their outputs are already exact-size;
the preference just confirms it) — both regression-tested intact.

Compile/clippy/test clean; live validation needs gnome-shell installed (then
`gnome-shell --headless` + m0 --source kwin-virtual with LUMEN_COMPOSITOR=mutter).

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

crates/lumen-host/src/vdisplay/gamescope.rs

@@ -44,6 +44,7 @@ impl VirtualDisplay for GamescopeDisplay {
             return Ok(VirtualOutput {
                 node_id,
                 remote_fd: None,
+                preferred_mode: Some((mode.width, mode.height, mode.refresh_hz)),
                 keepalive: Box::new(()),
             });
         }
@@ -66,6 +67,7 @@ impl VirtualDisplay for GamescopeDisplay {
         Ok(VirtualOutput {
             node_id,
             remote_fd: None,
+            preferred_mode: Some((mode.width, mode.height, mode.refresh_hz)),
             keepalive: Box::new(proc),
         })
     }