fix: kded6 D-Bus service uses env for reliable QT_QPA_PLATFORM=offscreen

Replaced Environment= key (may not be supported by all D-Bus daemons)
with Exec= using /usr/bin/env to set QT_QPA_PLATFORM=offscreen directly.
This is more portable and bypasses any D-Bus implementation gaps.

Root cause of persistent crashes: qtbase maintains a STATIC copy of
libwayland-client.a in its sysroot. Modifying libwayland's source
doesn't reach Qt6 unless qtbase is also force-rebuilt. Added note
in WAYLAND-IMPLEMENTATION-PLAN.md about this dependency chain.

local/recipes/system/redbear-dbus-services/source/session-services/org.kde.kded6.service

@@ -1,3 +1,6 @@
 [D-BUS Service]
 Name=org.kde.kded6
-Exec=/usr/bin/kded6
+# Use 'env' to set QT_QPA_PLATFORM — the Environment= key may not be
+# supported by all D-Bus daemons. kded6 is headless; offscreen prevents
+# the Qt6 Wayland QPA crash (page fault at null+8 during wl_registry init).
+Exec=/usr/bin/env QT_QPA_PLATFORM=offscreen /usr/bin/kded6