fix: make an existing classroom converge into the Plasma appliance

A real room upgraded to 0.1.45 and came up as stock Debian Plasma with
the old Waybar across the top, mako still catching notifications, and a
lock screen on an account that has no password.

The cause was one line, and it was in the lab: the harness handed the
session under test XDG_CONFIG_DIRS by hand, so every result it had ever
produced was a result about a machine somebody had already configured.
On a machine SDDM starts the session through PAM, PAM does not read
/etc/environment.d, and Debian's own plasma-workspace env.sh then set the
variable from scratch — so the classroom's settings directory was not in
the cascade at all. No layout, no lock policy, no Kiosk restrictions.

The settings now arrive the way Plasma delivers them, through a file in
/etc/xdg/plasma-workspace/env. Around that: the look and feel is
immutable on its own key so a stale profile cannot answer for it; the
palette is composed into the package, because naming a colour scheme does
not produce one and the dark bar had been coming from a technician's
hand; the desktop reset converges another desktop's appearance state and
keeps the room's own; and classroom-migrate stops, deactivates and masks
the old shell, including the D-Bus activation that brought mako back
while the panel was down — found only by killing the panel on a migrated
machine.

device.conf loses COMPOSITOR and comes off the hyprland channel, which
would otherwise have handed the room the experiment back.

The lab stops flattering the package: it restarts the machine, lets SDDM
actually autologin, and asks a fresh install and a machine upgraded from
the Hyprland package the same sixty questions. Both answer identically,
twice, and the panel still comes back from eight SIGKILLs.

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

docs/screenshots/README.md

@@ -77,6 +77,16 @@ container lab. See [../classroom-v2-experiment.md](../classroom-v2-experiment.md
 | `lock-panel-rmb` | right-clicking the bar: no menu |
 | `lock-krunner` | KRunner asked for, and not there |
 
+## The migration
+
+| | |
+| --- | --- |
+| `migration-before` | what a real room got when it upgraded: stock Debian Plasma, kickoff, a pager, a system tray, the Debian wallpaper. Reproduced in the lab by taking one line out of the harness. |
+| `migration-after` | the same machine after the fix — a Hyprland-era install, upgraded, rebooted |
+| `migration-after-bar` | its bar, at the size a teacher sees it |
+| `migration-parent-call` | Parent Call on the migrated machine, saying it cannot reach the church network rather than pretending it can |
+| `theme-light-live`, `theme-light-after-logout` | the room's own light theme, and the same room after logging out and back in. The first attempt at this pass lost it. |
+
 ## Failures, kept
 
 | | |