Split plan/ into topical files: vision (why), architecture (how), roadmap (when), atproto (glossary). Record this session's decisions: Zig+zat stack with headless core; one schema over two publicity modes (public records baseline / member-only spaces in phase 001); co-location principle; simulation rig on the zds fork; phase 001 lock on proposal 0016; M001 multi-account CLI; 3-digit id convention. Move the external lexicon style guide to ref/.