v0.5 — cleanup orphan code, add MIDI master volume learn, fix turn-deck crashes

- Remove vestigial lufs_meter_widget.py and debug_audio.py
- Delete unused WaveformWidget/WaveformMonitor classes, dead stubs, duplicate code
- Fix turn-mode deck crashes (deck3/deck4 missing from engine.decks, uninitialized manual_elapsed/duration)
- Replace raw print() with logging in main.py
- Add master_volume MIDI learn signal + settings popup entry
- Bump version to 0.5

cart_widget.py

@@ -234,12 +234,14 @@ class CartSlot(QFrame):
 
     def _refresh_display(self):
         deck = self.engine.decks[self.cart_key]
+        track = deck.track
+        playing = deck.playing
 
-        if not deck.playing and self._playing_icon:
+        if not playing and self._playing_icon:
             self._set_ui_stopped()
             self.status_label.setText("STOPPED")
 
-        if deck.track:
+        if track:
             self.elapsed_label.setText(deck.get_elapsed())
             self.remaining_label.setText(deck.get_remaining())