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

top_bar_widget.py

@@ -354,6 +354,7 @@ class SettingsPopup(QFrame):
             ("Master REL",        "master_control",   "release_ms"),
             ("Master MKG",        "master_control",   "makeup_gain_db"),
             ("Master CEIL",       "master_control",   "ceiling_db"),
+            ("Master Volume",     "master_volume",    "master"),
         ]
 
         self._learn_btns = {}