Overhaul: 5-task implementation

- Remove TURN decks (3/4), clean up MIDI, layout
- Single cart player with 3-item autoload queue + continue toggle
- Event logging (SessionLogger), mic tracking, large mic timer widget
- LUFS metering moved from external ports to internal master mix
- Master output WAV recording with top-bar REC toggle
- Music mix output (ch 3-8 only, no mics)
- Preview bar for playlist auditioning
- Merged show log with [SONG][CART][TALK] prefixes
- Live CSV writer for OBS song display (~/live_songs.csv)
- Misc fixes: preview race condition, datetime import for recording

midi_engine.py

@@ -354,7 +354,7 @@ class MidiEngine(QObject):
 
     def _handle_note_on(self, note: int):
         # Hardcoded Numark DJ2Go mapping (legacy)
-        NUMARK_CART = {0x44: 0, 0x43: 1, 0x46: 2, 0x45: 3}
+        NUMARK_CART = {0x44: 0}
 
         if note == 0x3B:
             self.deck_play_pause.emit('deck1')