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.

public/js/overlays.js

@@ -80,8 +80,18 @@ class HillshadeControl {
   _addLayer() {
     if (this._map.getSource(HILLSHADE_ID)) return;
 
+    // terrain.pmtiles is built from Mapterhorn (mapterhorn.com — see
+    // lib/maptool/pmtiles_builder.rb) whether generated in-app or by hand
+    // (docs/development.adoc), so this credits them regardless of how it
+    // got there — their own site lists the full per-region source
+    // provenance (CC-BY-4.0 and similar), not practical to reproduce here.
     const source = this._offlineAvailable
-      ? { type: "raster-dem", url: `pmtiles://${window.location.origin}/tiles/terrain.pmtiles`, encoding: "terrarium" }
+      ? {
+          type: "raster-dem",
+          url: `pmtiles://${window.location.origin}/tiles/terrain.pmtiles`,
+          encoding: "terrarium",
+          attribution: "Terrain: Mapterhorn",
+        }
       : {
           type: "raster-dem",
           tiles: ONLINE_TERRAIN_TILES,