feat: give every room the broker, one scale, and its own panel back

Little Waders took the Plasma build and reported what a container
cannot. A disc player on a machine with no optical drive, a clock nobody
at the back of the room could read, a browser at half the size of the
panel around it, and Parent Call with no broker to call.

The broker is the same for every classroom, so it is a default now and
not something to fill in room by room. classroom-migrate brings an
existing room to the same place without touching its name, its username
or its password, and Parent Call distinguishes a broker it cannot reach
from one that was never set up: those send somebody to look at
completely different things.

The disc player's detection was right and ran too late — alongside the
session rather than before it, and the panel is built from the layout the
moment a session starts. It runs before the display manager now, and
again from udev when a drive is plugged in.

UI_SCALE had a ceiling of 1.5, which cannot express a 4K television; a
clock pinned at 15 points, which is the same size at every scale; and a
browser that was never told the number at all. All three are fixed and
the room doubles together at 200%.

This also brings 0.1.47's work into the repository, where it had only
ever existed in a build directory under /tmp: the hidden display dialog,
its authenticated writer, the polkit action, and the church's own
wallpaper. One piece of it is deliberately left out. The custom volume
display needed a shell package of our own, and with one selected
plasmashell builds no panel at all — measured twice, with that build's
own files, on a classroom that deletes its panel configuration at every
login by design.

The lab connected to the real church broker the moment the package
defaulted to it. Nothing was left behind, which was checked, but the
guard that should have stopped it had never worked: it used iptables,
which the lab image does not have, and probed with /dev/tcp under dash,
where that always fails. Both reported success.

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

tools/lab/parent-call-rig

@@ -24,14 +24,19 @@ inlab() { docker exec "$NAME" "$@"; }
 guard() {
     # Container-local. This netns belongs to the container alone, so
     # nothing on the host or the church network is changed by it.
+    # Blackhole routes, not iptables: the lab image has no iptables at
+    # all, so these two lines had been failing silently since they were
+    # written. `tools/lab/lab up` now does the same thing at start-up, so
+    # this is a second statement of it rather than the only one.
     inlab sh -c '
-        iptables -C OUTPUT -d 10.30.0.0/16 -j REJECT 2>/dev/null ||
-            iptables -I OUTPUT -d 10.30.0.0/16 -j REJECT
-        iptables -C OUTPUT -d 10.40.0.0/16 -j REJECT 2>/dev/null ||
-            iptables -I OUTPUT -d 10.40.0.0/16 -j REJECT'
+        for net in 10.30.0.0/16 10.40.0.0/16; do
+            ip route replace blackhole "$net" || exit 1
+        done'
 
+    # bash, not sh. /dev/tcp is a bash feature and this probe reported
+    # every target unreachable under dash, whatever the network was doing.
     for target in 10.40.0.2/1883 10.30.0.2/1025; do
-        if inlab timeout 4 sh -c "exec 3<>/dev/tcp/${target}" 2>/dev/null; then
+        if inlab timeout 4 bash -c "exec 3<>/dev/tcp/${target}" 2>/dev/null; then
             echo "REFUSING TO CONTINUE: ${target} is still reachable" >&2
             exit 1
         fi