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

Protocol.cs

@@ -105,10 +105,6 @@ public class Command
     [JsonIgnore(Condition = JsonIgnoreCondition.WhenWritingNull)]
     public string? Channel { get; set; }
 
-    [JsonPropertyName("mode")]
-    [JsonIgnore(Condition = JsonIgnoreCondition.WhenWritingNull)]
-    public string? Mode { get; set; }
-
     [JsonPropertyName("value")]
     [JsonIgnore(Condition = JsonIgnoreCondition.WhenWritingNull)]
     public int? Value { get; set; }