Add interactive Periodic Table to Classroom
- New classroom module — interactive periodic table with all 118 elements, category filtering, hover preview, detail cards, and search
- New "Chemistry fundamentals" category in the classroom directory
Retired the legacy /codex/ archive
- Removed the /codex/ public route — all 140 entries were drafts and never published
- Spellbook now exposes a Source Batch filter (e.g. AI_Project_Research_2026-04)
- /codex/* URLs now 301-redirect to /spellbook/