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.

Gemfile.lock

@@ -95,16 +95,6 @@ GEM
     snaky_hash (2.0.7)
       hashie (>= 0.1.0, < 6)
       version_gem (~> 1.1, >= 1.1.14)
-    sqlite3 (2.9.6-aarch64-linux-gnu)
-    sqlite3 (2.9.6-aarch64-linux-musl)
-    sqlite3 (2.9.6-arm-linux-gnu)
-    sqlite3 (2.9.6-arm-linux-musl)
-    sqlite3 (2.9.6-arm64-darwin)
-    sqlite3 (2.9.6-x86-linux-gnu)
-    sqlite3 (2.9.6-x86-linux-musl)
-    sqlite3 (2.9.6-x86_64-darwin)
-    sqlite3 (2.9.6-x86_64-linux-gnu)
-    sqlite3 (2.9.6-x86_64-linux-musl)
     tilt (2.9.0)
     uri (1.1.1)
     version_gem (1.1.15)
@@ -132,7 +122,6 @@ DEPENDENCIES
   rubyzip (~> 2.4)
   sinatra (~> 4.2)
   sinatra-contrib (~> 4.2)
-  sqlite3 (~> 2.0)
 
 CHECKSUMS
   anonymous_loader (0.1.3) sha256=084a18e2439144d955447dc11dfc982f41fcd1583ad32d4d55151325dc44cb55
@@ -182,16 +171,6 @@ CHECKSUMS
   sinatra (4.2.1) sha256=b7aeb9b11d046b552972ade834f1f9be98b185fa8444480688e3627625377080
   sinatra-contrib (4.2.1) sha256=10d091c944d268aa910c618ea40a3c3ebe0533e6e32990d84af92235a3d26b4a
   snaky_hash (2.0.7) sha256=7d02c70012a3f932e48860cd024577908300c9aa615e0cb9b450aaa749cbcb4d
-  sqlite3 (2.9.6-aarch64-linux-gnu) sha256=d8b1f7d23efd7abac285775a9566562fc7debfef79d594e3a20354406fb7907c
-  sqlite3 (2.9.6-aarch64-linux-musl) sha256=3579e1c98cdc7ff5c3722847bb63ed4e1efb7ff675cb5e1e48ef2d4da5fb3bc9
-  sqlite3 (2.9.6-arm-linux-gnu) sha256=33541500e3615da02afe54a9cc38b17a6985d3cf9d8b76d6d0a83002f114e7ec
-  sqlite3 (2.9.6-arm-linux-musl) sha256=c5490af48bb228fefa54314e9541375c3907e70f8109f3881b5ff97e1c93ae33
-  sqlite3 (2.9.6-arm64-darwin) sha256=849b5d7f795e60fe25076d62c72dd722beb45b3850b516ad978d60ee848ec15b
-  sqlite3 (2.9.6-x86-linux-gnu) sha256=fbaa9f46f9708f57dd8a459b37fc269f9613e0cacf1547df01aa439cc45c20c0
-  sqlite3 (2.9.6-x86-linux-musl) sha256=6715026fbb5530e810b28ef43b9c4f84cd3c991f67b9d808a31fcc32b847abbd
-  sqlite3 (2.9.6-x86_64-darwin) sha256=b5842fea77781c14da03fa7bc0feb82db03a69e135affcb6f5399cbd2797a5f3
-  sqlite3 (2.9.6-x86_64-linux-gnu) sha256=613188ce02f614126ddbc38c5e217ccffd6306d0dcd9adca9764547aa890a634
-  sqlite3 (2.9.6-x86_64-linux-musl) sha256=d493b11818a3573387a1d56e1ee8fa00da23a683a7a1cc063e7a0feeed843abf
   tilt (2.9.0) sha256=da5735d0280bba96e9a91041bb14aee435ccad5c17b0fa519249ae543d9aa3a5
   uri (1.1.1) sha256=379fa58d27ffb1387eaada68c749d1426738bd0f654d812fcc07e7568f5c57c6
   version_gem (1.1.15) sha256=a73241587b29252e3567e0c818ded80730eb754d43b508f6ce4db22ca9ba27d0