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

midi_engine.py

@@ -19,7 +19,6 @@ NOTE_ON  = 0x90
 NOTE_OFF = 0x80
 CC       = 0xB0
 PITCH_BEND = 0xE0
-PITCH_BEND = 0xE0
 
 CONFIG_DIR = os.path.expanduser("~/.gti_radiostudio")
 CONFIG_FILE = os.path.join(CONFIG_DIR, "midi_map.json")
@@ -96,6 +95,7 @@ class MidiEngine(QObject):
     mixer_solo           = Signal(int, bool)
     mixer_pfl            = Signal(int, bool)
     master_control       = Signal(str, float)
+    master_volume        = Signal(float)
     controller_connected = Signal(bool, str)
     mapping_learned      = Signal(str, object, int, int, int, bool)
     raw_midi_event       = Signal(str, int, int, int)
@@ -115,7 +115,6 @@ class MidiEngine(QObject):
         self._last_cc_values = {}
         self._channel_volumes = {}
         self._master_knob_values = {}
-        self._master_knob_values = {}
 
     @property
     def learn_mode(self) -> bool:
@@ -303,6 +302,12 @@ class MidiEngine(QObject):
                     scaled = (127 - value if inv else value) / 127.0
                 self._master_knob_values[knob] = scaled
                 self.master_control.emit(knob, scaled)
+        elif action == 'master_volume':
+            if midi_type == PITCH_BEND:
+                scaled = (16383 - value if inv else value) / 16383.0
+            else:
+                scaled = (127 - value if inv else value) / 127.0
+            self.master_volume.emit(scaled)
 
     def _handle_note_on(self, note: int):
         # Hardcoded Numark DJ2Go mapping (legacy)