feat: M2 — gamescope virtual-display backend (spawn headless, capture its PipeWire node)

Third compositor on the VirtualDisplay seam. gamescope's model differs from KWin/Mutter: it's
not a runtime protocol but a micro-compositor we spawn — `gamescope --backend headless -W -H -r
-- <app>` — which composites at the client's size AND refresh natively (so no separate
refresh step), runs the app nested, and exports a built-in PipeWire node named "gamescope".
The backend spawns it, discovers that node via pw-dump, and returns a VirtualOutput whose
keepalive owns the process (drop = kill = teardown). App via LUMEN_GAMESCOPE_APP. Select with
LUMEN_COMPOSITOR=gamescope; m0's virtual source now honors LUMEN_COMPOSITOR so any backend is
testable without a client. Input (gamescope's libei/EIS socket) is a follow-up.

Builds/clippy/fmt clean. Needs gamescope installed to validate; headless capture on the
proprietary NVIDIA driver is plausible-by-architecture but unproven — validate empirically.

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

crates/lumen-host/Cargo.toml

@@ -54,6 +54,8 @@ wayland-protocols-misc = { version = "0.3", features = ["client"] }
 # `wayland-backend` is referenced by the generated interface tables.
 wayland-scanner = "0.31"
 wayland-backend = "0.3"
+# Parse `pw-dump` JSON to find gamescope's PipeWire node (gamescope backend).
+serde_json = "1"
 # Builds/validates the xkb keymap uploaded to the virtual keyboard + tracks modifier state.
 xkbcommon = "0.8"
 # Opus encode for the GameStream audio stream (links system libopus).