Historical heatmap (scaffold)

TSP-evaluation heatmap window May 12–25, 2026. Road-keyed segments, route-agnostic, between consecutive signalized intersections; TSP-zone halo for segments inside the Fullerton city boundary. Loads tile JSON from /fixtures/octa-heatmap.json (locked schema, see briefs/frontend-and-heatmap.md §7.10) until the aggregator's production tile lands.