Add 9 new poster PDFs and update publications spreadsheet

Added PDFs for posters from multiple conferences:
- Wetterhahn 2025: Peng et al. (EEG knowledge tracking)
- OHBM 2022: Saggar et al. (Higher-order brain interactions)
- CCN 2022: Jain et al. (Cognitive markers mental health)
- CEMS 2020: Owen et al. (ECoG Gaussian process), Owen & Manning (Brain complexity)
- CEMS 2021: Xu et al. (Past/future inference)
- CEMS 2023: Xu & Manning (Statistical mechanics time)
- CEMS 2025: Xu & Manning (Temporal asymmetries)
- SfN 2021: Ziman & Manning (False familiarity)

Also added PDF links to existing Wetterhahn 2023/2024 entries.

Total publications: 122 (58 posters)

🤖 Generated with [Claude Code](https://claude.com/claude-code)

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

Author: jeremymanning