feat: add PDFs for 3 of 4 new publications

- luo2026federated.pdf (arXiv preprint)
- yang2026morepair.pdf (arXiv preprint)
- dramko2026idioms.pdf (author website)

Still missing (no public PDF): mamede2025ase.pdf (ASE 2025)

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

Author: clegoues