Cleanup pass: remove dead code, fix bugs, simplify

Dead code removed:
- MusicAnalyzer.BuildWaveFrame (replaced by DrumDetector + BuildMelodyFrame)
- MusicAnalyzer.ExtractFeatures simplified to melody-only (removed rhythm band)
- MusicAnalyzer.RhythmLow/RhythmHigh constants, TickFeature.RhythmFlux
- LiveCapture._liveMaxFlux + _prevRhythmMag (computed, never read)
- State.ActualA/ActualB, State.LastSentA/LastSentB (set, never read)
- Program.cs StrengthChanged handler (only wrote to dead fields)
- Command.Mode property (never referenced)
- DrumDetector.FreqKick/Snare/Brass/Silent static arrays (unused)

Bugs fixed:
- DrumDetector.BuildFrame: freqBytes was byte[16], now byte[4]
- CoyoteDevice fallback: use nameFilter param instead of hardcoded string

Refactoring:
- Server.DoStatus calls BuildStatusPush(null) instead of duplicating
- HeatMap: removed redundant colW assignment
- Removed unused System.Numerics imports from LiveCapture, MusicAnalyzer

HeatMap.cs

@@ -37,7 +37,6 @@ public class HeatMap : Panel
 
         int w = Width;
         int h = Height;
-        int colW = Math.Max(1, w / _maxTicks);
 
         // Draw axis labels
         using var font = new Font(FontFamily.GenericMonospace, 7);
@@ -48,7 +47,7 @@ public class HeatMap : Panel
         int plotX = 36;
         int plotW = w - plotX - 4;
         int plotH = h - 4;
-        colW = Math.Max(1, plotW / _maxTicks);
+        int colW = Math.Max(1, plotW / _maxTicks);
 
         for (int i = 0; i < _history.Count; i++)
         {