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>

Cargo.lock

@@ -1528,9 +1528,11 @@ dependencies = [
  "tokio",
  "tracing",
  "tracing-subscriber",
+ "wayland-backend",
  "wayland-client",
  "wayland-protocols-misc",
  "wayland-protocols-wlr",
+ "wayland-scanner",
  "x509-parser",
  "xkbcommon",
 ]