Building Chapter 7 — a sandbox and a live NWS panel that share one set of formulas

How heat-metrics-lab Chapter 7 got two interactive surfaces — manual sliders and a live api.weather.gov fetch — wired to the same drift-gated metrics.js. Covers the playground skill's single-state-object pattern, the CSP-respecting ZIP-to-lat/lng pivot, a two-stage NWS fetch, and a solar-elevation estimator built from first principles.