fix: deliver test notifications into the classroom session

The classroom desktop belongs to the classroom user and mako sits on
that user's D-Bus session. An administrator over SSH is in a different
session where nothing owns org.freedesktop.Notifications, so notify-send
asked D-Bus to start a second mako, which had no display to open and
timed out with NoReply.

Find the running sway process for the classroom user and deliver into
its session instead. XDG_RUNTIME_DIR and the bus address are read from
that process, falling back to /run/user/<uid> and its bus socket when
the environment cannot be read — root has ptrace access on the endpoint,
but that is not somewhere to be brittle. The Wayland display is read the
same way, and failing that from the wayland-N.lock sway holds open, so
no socket name is ever assumed. notify-send only needs the bus, so a
display that cannot be resolved is not worth refusing to send over.

Refuses to send when mako is not already running, rather than letting
D-Bus start one, and says plainly what is wrong when there is no
session, no privileges, or no bus socket. --dry-run reports what it
resolved without sending.

Behaviour inside the session is unchanged, and the sample notifications
and styling are untouched.

The tests stand a real background process in for sway so /proc/<pid>/environ
is genuine rather than mocked, and cover both invocation paths plus the
fallbacks and every refusal. The build runs them.

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

docs/testing.md

@@ -56,6 +56,28 @@ hook. Minimising is the dock's Browser button or `Super+M`.
       renders in the critical style and does not time out
 - [ ] Notifications appear over a fullscreen video
 
+Over SSH from an administrator account, run it with sudo:
+
+```
+sudo classroom-notify-test --dry-run   # show the session it resolved
+sudo classroom-notify-test             # send into the classroom desktop
+```
+
+The classroom desktop belongs to the classroom user, and mako sits on
+that user's D-Bus session. A plain notify-send from an SSH session lands
+in the administrator's own session, where nothing owns
+org.freedesktop.Notifications, so D-Bus tries to start a second mako with
+no display and the call times out with NoReply. The script finds the
+running sway process for the classroom user and delivers into its
+session instead. It never starts mako.
+
+- [ ] `sudo classroom-notify-test` from an SSH session displays on the
+      classroom screen
+- [ ] `sudo classroom-notify-test --dry-run` reports the resolved
+      XDG_RUNTIME_DIR, bus address and Wayland display
+- [ ] Running it as an unprivileged admin account explains it needs sudo
+- [ ] `tests/test-classroom-notify-test.sh` passes (also run by the build)
+
 ## DVD
 
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