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/00_nulld.service

@@ -0,0 +1,8 @@
+[unit]
+description = "/dev/null"
+default_dependencies = false
+
+[service]
+cmd = "zerod"
+args = ["null"]
+type = { scheme = "null" }