Design written after the spike ran (controller v0.247.0 live on demo-hp): mechanism,
flow, fallback, gates per language, catalog model, sliced plan with costs, operator
rulings 1-4 recorded, CC decisions 5-6, open decisions 1b and 7.
Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_0159rPz1ZhFKsS53msqPYxtS
