feat(block18.5): agent auto-resign when position is hopeless for 3+ turns

stage: commit in-progress Block 4 work required for HF deployment

Phase 4.0–4.3: demo prep — auth, Elo fairness, rooms, lobbies, conversational entry, evolution

Phase 2a hotfix: live gameplay works end-to-end

Phase 2a engines: base types, registry, Maia-2, Stockfish, board abstraction