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/main.rs

@@ -145,7 +145,10 @@ fn parse_m0(args: &[String]) -> Result<Options> {
                 source = match next()?.as_str() {
                     "synthetic" => Source::Synthetic,
                     "portal" => Source::Portal,
-                    other => bail!("unknown --source '{other}' (synthetic|portal)"),
+                    "kwin-virtual" => Source::KwinVirtual,
+                    other => {
+                        bail!("unknown --source '{other}' (synthetic|portal|kwin-virtual)")
+                    }
                 }
             }
             "--width" => {
@@ -223,7 +226,9 @@ USAGE:
     lumen-host m0 [OPTIONS]       M0 capture→encode→file pipeline spike
 
 OPTIONS:
-    --source <synthetic|portal>  frame source (default: portal)
+    --source <synthetic|portal|kwin-virtual>
+                                 frame source (default: portal). 'kwin-virtual' creates a
+                                 KWin virtual output at --width x --height and captures it
     --seconds <N>                capture duration in seconds (default: 5)
     --fps <N>                    target frame rate (default: 60)
     --codec <h264|h265|av1>      NVENC codec (default: h265)