Build terrain.pmtiles from Mapterhorn instead of fetching tiles ourselves

Mapterhorn (mapterhorn.com, Protomaps-affiliated) publishes pre-built
Terrarium-encoded terrain as PMTiles: a global planet.pmtiles (z0-12)
plus ~450 regional chunks (z13+). Same pmtiles extract --bbox=...
mechanism offline.pmtiles already uses, just against their files —
confirmed live against their real manifest and the documented
Interlaken example. Replaces fetching every individual tile ourselves
(even after this session's earlier concurrency fix, still tens of
minutes for a real region) with 1-3 extract calls plus a merge when
the bbox spans more than one chunk: seconds, not minutes.

Removes CONCURRENT_TILE_FETCHES, write_terrarium_mbtiles, tile_range,
lng/lat_to_tile_x/y, fetch_terrarium_tile, and the sqlite3 gem
entirely — none of it is needed once the tiles arrive pre-packaged.

dem.js's point-elevation lookup (shadow.js's terrain-shadow overlay)
hardcoded a 256px PNG tile assumption that Mapterhorn's 512px WebP
tiles would have silently broken — now reads the actually-decoded
tile's own size and mime type instead of assuming either.

docs/third-party-notices.adoc

@@ -82,6 +82,15 @@ The online map style (not vendored, fetched at runtime when the
 https://tiles.openfreemap.org/styles/liberty[OpenFreeMap "liberty"],
 built from https://www.openstreetmap.org[OpenStreetMap] data.
 
+`terrain.pmtiles` (`lib/maptool/pmtiles_builder.rb`'s in-app
+regeneration, or the manual recipe in `docs/development.adoc`) is
+extracted at build time, not vendored, from
+https://mapterhorn.com[Mapterhorn] — a Protomaps-affiliated project
+publishing pre-built Terrarium-encoded terrain as PMTiles. Per-region
+source licenses vary (CC-BY-4.0 and similar); see
+https://download.mapterhorn.com/attribution.json[their own attribution
+list] for the full provenance.
+
 Not a frontend asset like the above, but also baked into this app's own
 Docker image (not a separate container the way kiwix-serve/gpsd are —
 see `docs/deployment.adoc`/`docs/development.adoc`): the `pmtiles` CLI