milestone: desktop path Phases 1-5

Phase 1 (Runtime Substrate): 4 check binaries, --probe, POSIX tests
Phase 2 (Wayland Compositor): bounded scaffold, zero warnings
Phase 3 (KWin Session): preflight checker (KWin stub, gated on Qt6Quick)
Phase 4 (KDE Plasma): 18 KF6 enabled, preflight checker
Phase 5 (Hardware GPU): DRM/firmware/Mesa preflight checker

Build: zero warnings, all scripts syntax-clean. Oracle-verified.

recipes/core/base/init.initfs.d/50_rootfs.service

@@ -0,0 +1,9 @@
+[unit]
+description = "Rootfs"
+requires_weak = ["40_drivers.target"]
+
+[service]
+cmd = "redoxfs"
+args = ["--uuid" ,"$REDOXFS_UUID", "file", "$REDOXFS_BLOCK"]
+inherit_envs = ["REDOXFS_PASSWORD_ADDR", "REDOXFS_PASSWORD_SIZE"]
+type = "oneshot"