Replace render_full_set with stem-based version using demucs + stem_mixer\n\nThe render function now:\n1. Loads demucs htdemucs model\n2. Separates each track into drums/bass/vocals/other stems\n3. Uses stem_mixer.mix_stems for surgical transitions\n4. Falls back to mixer.mix_set if demucs unavailable\n\nThis ensures zero kick clashing during transitions."
Patch render_full_set to use demucs stem separation + stem_mixer\n\nAdds import at top and overrides render_full_set to:\n1. Run demucs htdemucs on each track\n2. Use stem_mixer.mix_stems for surgical drum/bass transitions\n3. Falls back to filter-based mixer if demucs fails"