Sync the map view to the URL, like Google Maps/OSM permalinks

Panning/zooming now updates #map=<zoom>/<lat>/<lon> in the URL, and loading
a URL with one already opens at that view instead of the default center —
so copying the URL bar shares the exact spot someone's looking at, and a
page reload doesn't lose the current view either.

This is MapLibre GL JS's own built-in Hash control (hash: "map" on the Map
constructor) — it already does exactly this, replaceState and all, no
custom code needed.

docs/architecture.adoc

@@ -103,6 +103,18 @@ No PMTiles file ships with the repository — it is expected to be
 supplied locally (see `docs/development.adoc`) and stays gitignored
 because such extracts are typically large binary files.
 
+== Map permalink
+
+The map's `hash: "map"` option (`public/js/map.js`) turns on MapLibre
+GL JS's built-in `Hash` control — nothing hand-rolled here. It keeps
+`#map=<zoom>/<lat>/<lon>` in the URL in sync with the current view (via
+`history.replaceState`, so panning never spams the back button) and,
+if the page loads with one already in the URL, opens there instead of
+the default `CENTER`/`ZOOM` — the same permalink pattern OSM and Google
+Maps use, so copying the URL bar shares the exact spot someone's
+looking at, and reloading the page doesn't reset the view back to the
+default.
+
 == Weather radar overlay
 
 `public/js/radar.js` adds a "Radar" map control that overlays recent