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]>

config/plasma/xdg/kdeglobals

@@ -32,7 +32,12 @@ action/help_report_bug=false
 action/help_about_kde=false
 action/help_contents=false
 
-[General][$i]
+# Not immutable. Marking this group [$i] froze the colour scheme so hard
+# that `classroom-theme light` changed the running session and then lost
+# the change at the next start — measured. These are defaults for a room
+# that has no settings application anyway; the lock belongs on the
+# restrictions above, not on the palette.
+[General]
 ColorScheme=EkklesiaDark
 AccentColor=79,179,168
 # The teacher's own words are not decoration: keep text rendering plain
@@ -41,13 +46,13 @@ XftAntialias=true
 XftHintStyle=hintslight
 XftSubPixel=rgb
 
-[KDE][$i]
+[KDE]
 widgetStyle=Breeze
 LookAndFeelPackage=org.ekklesia.classroom.desktop
 SingleClick=false
 AnimationDurationFactor=0.5
 
-[Icons][$i]
+[Icons]
 Theme=breeze-dark
 
 [KFileDialog Settings][$i]