UI v3: fintech-direction design pass (5 grafts), waterfall historical-mode fix, explain-strip placement + shared hook, palette contrast fix

Ship App v2 + Tier-2 sandbox (5-tab UI, analyze_data, contract-first API)

UI: action-effect waterfall + header updates + stage-horizon guide (WaterfallChart.jsx)

Day 4: runtime file set (engine, agent, API, UI source, panel, DFAST scenarios, model cache, exhibits)