30-Line Fix for Spotify's Half-Time BPM Detection Flaw

Essentia's RhythmExtractor2013 reports Blinding Lights at 85.4 BPM instead of 171 BPM due to a known tactus-level ambiguity in beat-tracking algorithms. The author presents a 30-line heuristic that detects half-time errors by checking BPM range, onset rate, energy, and danceability. This fix corrects a common failure mode affecting modern pop and trap-influenced tracks.