Use local PMTiles basemap

docs/vector-tiles.md

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+# Mandan vector tiles
+
+The yard map reads the committed `tiles/mandan.pmtiles` archive directly in
+the browser using MapLibre GL JS. No tile server or internet-hosted raster tile
+service is required. The map omits the Protomaps symbol layers so it does not
+need remotely hosted font glyphs or sprites; the basemap is fully local.
+
+The archive covers Mandan and a modest surrounding margin:
+`-101.05,46.74,-100.75,46.93`. PMTiles extraction is tile-aligned, so the
+actual archive includes a little neighboring context.
+
+## Build an update
+
+Install the [`pmtiles` CLI](https://docs.protomaps.com/pmtiles/cli), choose a
+dated Version 4 daily build from
+[`maps.protomaps.com/builds`](https://maps.protomaps.com/builds), and run:
+
+```bash
+scripts/extract-mandan-pmtiles.sh \
+  https://build.protomaps.com/YYYYMMDD.pmtiles
+```
+
+The script refuses to overwrite an existing archive. Move the old archive
+aside before deliberately replacing it.