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.

docs/development.adoc

@@ -71,10 +71,10 @@ and similar, varying by region):
 
 [source,shell]
 ----
-pmtiles extract https://download.mapterhorn.com/planet.pmtiles planet-extract.pmtiles --bbox=west,south,east,north --maxzoom=15 --overfetch=0
+pmtiles extract https://download.mapterhorn.com/planet.pmtiles planet-extract.pmtiles --bbox=west,south,east,north --maxzoom=13 --overfetch=0
 # only if a regional chunk covers the bbox too (see download_urls.json) —
 # most bboxes hit exactly one, a bbox straddling a chunk boundary hits two:
-pmtiles extract https://download.mapterhorn.com/<chunk>.pmtiles chunk-extract.pmtiles --bbox=west,south,east,north --maxzoom=15 --overfetch=0
+pmtiles extract https://download.mapterhorn.com/<chunk>.pmtiles chunk-extract.pmtiles --bbox=west,south,east,north --maxzoom=13 --overfetch=0
 pmtiles merge planet-extract.pmtiles chunk-extract.pmtiles data/terrain.pmtiles
 ----
 
@@ -88,6 +88,14 @@ fragile mega-request for many small ones instead (spread over
 connection only loses one small request's worth of progress, not the
 whole extract.
 
+`--maxzoom=13`: raster DEM tiles don't compress away like vector ones do
+(every tile is a real image, even over flat/empty terrain), so each extra
+zoom level roughly quadruples storage — the app's own build
+(`PmtilesBuilder::TERRAIN_MAX_ZOOM`) caps at 13, each regional chunk's
+own native minzoom, rather than the online hillshade layer's z15. A
+12-country selection at z15 was headed well past 181GB; z13 is still
+plenty for the hillshade toggle and the planned slope readout.
+
 Tiles are 512px WebP, not the 256px PNG this app's *online* Terrarium
 source uses — `public/js/dem.js`'s own point-elevation lookups
 (`shadow.js`'s terrain-shadow overlay) read the actual decoded tile