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

State.cs

@@ -15,8 +15,6 @@ public class State
     public readonly ConcurrentQueue<WaveFrame> StreamA = new();
     public readonly ConcurrentQueue<WaveFrame> StreamB = new();
 
-    public int ActualA, ActualB;
-    public int LastSentA, LastSentB;
     public double LimitScaleA, LimitScaleB;
     public byte[]? LastFreqA, LastIntA, LastFreqB, LastIntB;
     public bool LastActiveA, LastActiveB;
@@ -133,8 +131,6 @@ public class State
                     : (byte)Math.Clamp(desired, 0, max);
             var valA  = ApplyLimit(DesiredA, MaxA, LimitModeA);
             var valB  = ApplyLimit(DesiredB, MaxB, LimitModeB);
-            LastSentA = valA;
-            LastSentB = valB;
             LimitScaleA = DesiredA > 0 ? (double)valA / DesiredA : 0;
             LimitScaleB = DesiredB > 0 ? (double)valB / DesiredB : 0;
             DirtyA = false;
@@ -167,7 +163,6 @@ public class State
                 (modeA, modeB) = (modeB, modeA);
                 (freqA, freqB) = (freqB, freqA);
                 (intA, intB) = (intB, intA);
-                (LastSentA, LastSentB) = (LastSentB, LastSentA);
                 (LastFreqA, LastFreqB) = (LastFreqB, LastFreqA);
                 (LastIntA, LastIntB) = (LastIntB, LastIntA);
                 (LastActiveA, LastActiveB) = (LastActiveB, LastActiveA);