Stems-first onset detector — splits audio into vocals/drums/bass/other via Demucs, then runs onset detection on a single channel (not the muddy full mix) to emit a clean beatlib BeatEvent sidecar. Channel-locked cuts: kick-only, syllable-only, sub-only. Built on top of @foenem_jarvis/beatlib v1.0.0 and registers itself in beatlib.DETECTORS as "stems-beatcut" so consumers can dispatch by name. Sister LyricEvent emitter via whisper. Drop-in producer for feverdream, beatcut, transition-engine. Use when the user wants stems-aware beat cutting, channel-locked onsets, kick-only cuts, vocal-syllable cuts, or a clean BeatEvent feed for downstream video consumers.