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

icons.py

@@ -312,39 +312,6 @@ def icon_cog(color: str = "#aaaaaa") -> QIcon:
     return _make_icon(paint, color)
 
 
-def icon_cog(color: str = "#aaaaaa") -> QIcon:
-    def paint(p, r, c):
-        pen = QPen(c, r.width() * 0.1)
-        pen.setCapStyle(Qt.RoundCap)
-        p.setPen(pen)
-        p.setBrush(Qt.NoBrush)
-        # Outer circle (gear ring)
-        cx, cy = r.center().x(), r.center().y()
-        outer_r = min(r.width(), r.height()) * 0.42
-        inner_r = outer_r * 0.6
-        # Draw gear teeth (6 bumps)
-        import math
-        p.setBrush(QColor(c))
-        for i in range(6):
-            angle = i * 60 - 30
-            a_rad = math.radians(angle)
-            bx = cx + outer_r * math.cos(a_rad)
-            by = cy + outer_r * math.sin(a_rad)
-            tooth = QPainterPath()
-            tw = r.width() * 0.12
-            th = r.width() * 0.18
-            tooth.addRect(bx - tw/2, by - th/2, tw, th)
-            p.drawRect(bx - tw/2, by - th/2, tw, th)
-        # Center circle
-        p.setPen(Qt.NoPen)
-        p.setBrush(QColor(c))
-        p.drawEllipse(QPointF(cx, cy), inner_r, inner_r)
-        # Inner hole
-        p.setBrush(QColor("#000000"))
-        p.drawEllipse(QPointF(cx, cy), inner_r * 0.55, inner_r * 0.55)
-    return _make_icon(paint, color)
-
-
 def icon_theme(color: str = "#00ff41") -> QIcon:
     def paint(p, r, c):
         p.setPen(Qt.NoPen)