Three fixes for the KWin DRM device discovery failure:
1. drm_scheme_ready(): replace head -c 1 with exec 3< open test.
Reading from a DRM scheme fd blocks because the scheme expects
ioctl-style request/response, not streaming reads. Use open()
success as the scheme availability probe instead.
2. ConsoleKitSession::create(): return nullptr immediately.
The D-Bus isServiceRegistered() call can block indefinitely when
the bus daemon doesn't fully implement org.freedesktop.DBus.
With both LogindSession and ConsoleKitSession returning nullptr,
Session::create() falls through to NoopSession which uses plain
open() for DRM device access.
3. Boot chain deps: redox-drm depends on driver-manager,
greeter depends on evdevd (keyboard/mouse ready before login).
Also includes: KF6 CMake build fixes, Qt6 platform patches,
libdrm Redox ioctl shim, and wayland.toml scheme check fix.
