feat: M2 — KWin virtual-output backend behind a VirtualDisplay trait (native client resolution)

Honor the client's requested resolution by rendering a compositor virtual output at
exactly that size — native, headless, no scaling. There is no cross-compositor Wayland
protocol for this, so it's a per-compositor backend behind the (previously stubbed)
VirtualDisplay trait.

- vdisplay.rs: VirtualDisplay::create(mode) now returns a live VirtualOutput
  { node_id, remote_fd: Option<OwnedFd>, keepalive } with RAII teardown (drop releases
  the output) instead of an inert OutputHandle + explicit destroy. Add compositor
  detect() (LUMEN_COMPOSITOR / XDG_CURRENT_DESKTOP).
- vdisplay/kwin.rs: the KWin backend — the zkde_screencast_unstable_v1 stream_virtual_output
  client (vendored protocol XML + wayland-scanner codegen). Creates a WxH output, returns
  its PipeWire node (default daemon, remote_fd=None); a keepalive thread holds the Wayland
  connection until dropped. (Moved here from capture/kwin.rs — it's a vdisplay backend, not
  capture.)
- capture: generalize the PipeWire consumer to Option<OwnedFd> (portal remote vs. default
  daemon) and add capture_virtual_output(vout), compositor-agnostic, owning the keepalive.
- gamestream/stream.rs: LUMEN_VIDEO_SOURCE=virtual creates a virtual display sized to the
  client's cfg and captures it (self-contained, not pooled — a reconnect at a new
  resolution gets a fresh output).
- m0: --source kwin-virtual goes through the trait.

Verified end-to-end against the running headless KWin: the request reaches the compositor
and is handled cleanly. Native creation needs a backend implementing createVirtualOutput —
the DRM backend, or the VirtualBackend since KWin 6.5.6; on this box's --virtual 6.4.5 it
returns "Could not find output" (expected; validates after the KWin upgrade). wlroots/Mutter
backends are the next ones to land on the same seam.

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

crates/lumen-host/src/m0.rs

@@ -24,6 +24,9 @@ pub enum Source {
     Synthetic,
     /// Live monitor via the xdg ScreenCast portal + PipeWire.
     Portal,
+    /// KWin virtual output created at `width`x`height` (zkde_screencast). Lets us validate
+    /// capture (and zero-copy) at an arbitrary client resolution against a headless KWin.
+    KwinVirtual,
 }
 
 #[derive(Clone, Debug)]
@@ -57,6 +60,23 @@ pub fn run(opts: Options) -> Result<()> {
             tracing::info!("M0 source: xdg ScreenCast portal (live monitor)");
             capture::open_portal_monitor().context("open portal capturer")?
         }
+        Source::KwinVirtual => {
+            tracing::info!(
+                width = opts.width,
+                height = opts.height,
+                "M0 source: KWin virtual output (zkde_screencast)"
+            );
+            let mut vd = crate::vdisplay::open(crate::vdisplay::Compositor::Kwin)
+                .context("open KWin virtual display")?;
+            let vout = vd
+                .create(lumen_core::Mode {
+                    width: opts.width,
+                    height: opts.height,
+                    refresh_hz: opts.fps,
+                })
+                .context("create KWin virtual output")?;
+            capture::capture_virtual_output(vout).context("capture virtual output")?
+        }
     };
 
     // Activate the capturer so the portal/PipeWire process callback actually delivers frames