TerrainRegionExtractor

Extracts a region's offline terrain: a global low-res layer (always, up to MapterhornEndpoints.GLOBAL_MAX_ZOOM) plus, where bounds fits inside a single z6 tile, a regional high-res layer on top — ported from the sibling iOS app's two-tier TerrainStore download behavior. One tile per HTTP request, like the base offline layer's own extractor; see its README for why that bounds MAX_TILES well below iOS's own per-region cap.

This module owns nothing about how a terrain download relates to a base map region (shared storage budget, region lifecycle, "delete the terrain without touching the basemap") — that's the caller's integration concern, same separation as TerrainTileStore only knowing about the directory it's handed.