Lower offline terrain to z13, not z15 — raster DEM storage explodes with zoom

The 12-country selection's terrain.pmtiles was headed well past the
181GB an earlier 5-country build already produced at z15. Raster DEM
tiles don't compress away like vector ones (every tile is a real
image, even over empty terrain), so each extra zoom level roughly
quadruples storage. z13 is each Mapterhorn regional chunk's own native
minzoom — one raster level per chunk instead of a 15-13=2-level
pyramid on top — still enough for the hillshade toggle and the planned
slope readout. Online hillshade keeps its own z15 cap; this only
affects the offline copy.

lib/maptool/pmtiles_builder.rb

@@ -88,11 +88,15 @@ module Maptool
     # archive" story via go-pmtiles, unrelated to the basemap swap.
     MAPTERHORN_MANIFEST_URL = "https://download.mapterhorn.com/download_urls.json"
     MAPTERHORN_PLANET_NAME = "planet.pmtiles"
-    # Matches the online hillshade layer's own cap (public/js/overlays.js's
-    # ONLINE_TERRAIN_TILES source, maxzoom: 15) — applied uniformly even
-    # though some Mapterhorn chunks natively offer more, so the offline
-    # experience doesn't vary in detail from one region to the next.
-    TERRAIN_MAX_ZOOM = 15
+    # Deliberately *below* the online hillshade layer's own cap
+    # (public/js/overlays.js's ONLINE_TERRAIN_TILES source, maxzoom: 15) —
+    # raster DEM tiles don't compress away like vector ones do, so every
+    # extra zoom level roughly quadruples storage. z13 is each regional
+    # chunk's own native minzoom (see the comment above), so this is the
+    # cheapest useful cut: one raster level per chunk, not a 15-13=2-level
+    # pyramid on top of it. Bumped into a 181GB terrain.pmtiles for a
+    # 5-country selection before this was lowered — see docs/development.adoc.
+    TERRAIN_MAX_ZOOM = 13
 
     OFFLINE_STATUS = { state: "idle" }
     TERRAIN_STATUS = { state: "idle" }