build: make the classroom desktop something a .deb installs

Building the package found two things reading the tree did not.

dpkg refused it outright: plasma-workspace already owns
/etc/xdg/plasmanotifyrc and friends, and two packages cannot own one
path. The room's settings move to a directory of their own with
/etc/environment.d putting it in front of /etc/xdg, which is KConfig's
own cascade rather than a file fight.

The postinst then failed on a chmod for a command that no longer
exists. It now walks what the package installed instead of naming
files, removes the login shell that used to start sway on tty1, and
enables SDDM and the audio service.

Verified by installing the built package on a fresh machine and
starting the session from it: restrictions in force, palette applied,
audio service enabled and running, panel supervision unconditional.

Co-Authored-By: Claude Opus 5 <[email protected]>

tests/test-deployment-support.sh

@@ -92,6 +92,20 @@ check("restart is unconditional", "Restart=always" in supervision)
 check("the burst limit is lifted", "StartLimitIntervalSec=0" in supervision)
 check("the drop-in is packaged", "plasma-plasmashell.service.d" in build)
 
+print()
+print("4b2. the session settings do not fight another package")
+# plasma-workspace owns /etc/xdg/kdeglobals and friends. dpkg refused the
+# package when the classroom tried to own them too, so the room's copies
+# live in a directory of their own, in front of /etc/xdg.
+check("settings install to the classroom's own directory",
+      "$STAGE/etc/ekklesia-classroom/xdg/" in build)
+check("nothing is installed into /etc/xdg", "$STAGE/etc/xdg/" not in build)
+check("and that directory is searched first",
+      "XDG_CONFIG_DIRS=/etc/ekklesia-classroom/xdg:/etc/xdg" in
+      (root / "config/environment/90-ekklesia-classroom.conf").read_text())
+check("the environment file is packaged",
+      "/etc/environment.d/90-ekklesia-classroom.conf" in build)
+
 print()
 print("4c. the teacher cannot reach a command line")
 for restriction in ("run_command=false", "shell_access=false",