feat: decide the classroom's audio output from what is plugged in

The room that prompted this had a television on HDMI with working PCM
audio and nothing in the analog jack, and WirePlumber chose analog. A
teacher who corrected it by hand then had to restart the browser before
any sound came out.

The policy is a pure function so it can be tested against that room
rather than against a machine: an unavailable port is not a candidate,
an analog port merely existing is not evidence of a cable, and an
explicit AUDIO_OUTPUT overrides the lot. Twelve cases, the real room
among them.

The daemon follows PipeWire's own event stream rather than polling, and
moves streams that are already playing, which is the part that stops a
teacher having to close and reopen the browser.

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

tools/lab/lab

@@ -150,6 +150,15 @@ click)
     sleep 0.5
     ;;
 
+rclick)
+    outer swaymsg seat - cursor set "$2" "$3" >/dev/null
+    sleep 0.4
+    outer swaymsg seat - cursor press button3 >/dev/null
+    sleep 0.15
+    outer swaymsg seat - cursor release button3 >/dev/null
+    sleep 0.5
+    ;;
+
 move)
     outer swaymsg seat - cursor set "$2" "$3" >/dev/null
     ;;