Collect browser-side worker round-trips, computation times, main-thread
operations, and feature counts, batch them client-side, and expose as
Prometheus histograms via a new POST /api/perf endpoint.
R-tree building, hex grid generation, and percentile sorting now run
off the main thread, eliminating 20s+ UI freezes on large datasets.
The old bundled HexgridHeatmap.js is replaced by a typed worker +
main-thread client with dual R-trees (worker for grid gen, main
thread for synchronous click queries).
