Add scholarly references for classifier models

Add references and BibTeX citations for:
- Music loop instrument role (Freesound Loop Dataset)
- Nsynth acoustic/electronic, bright/dark, instrument, reverb
- Mood Acoustic and Mood Electronic (Laurier et al. 2008)
- MagnaTagATune (Law et al. 2009)
- Million Song Dataset (Bertin-Mahieux et al. 2011)

Also fix GitHub link emoji consistency (use 💻 for all GitHub links).

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

Author: palonso