geo: acquire the 3DEP 1 m DEM window in EPSG:26911

550x350 m clipped to the block, already in the project CRS, 984 KB, committed
through LFS. Verified against the service's own point query and the earlier
probe: 115.911/119.422/117.593 m, agreement within 6 cm.

Adds Geo/scripts/dem_stats.py, a stdlib tiled-TIFF reader, because GDAL cannot
run on the host and the exported raster is tiled rather than strip-organised.

THIRD_PARTY_NOTICES.md

@@ -21,7 +21,7 @@ Status values: **verified** (licence confirmed and permits our use), **pending**
 | --- | --- | --- | --- |
 | OSM extract — Hollywood block, 2026-11-06 | OpenStreetMap API v0.6, `Geo/raw/osm-api-map-2026-11-06.osm` | ODbL 1.0 | verified — manifest and SHA-256 in `Geo/manifests/` |
 | OSM building ways 424024147, 424024148 | OpenStreetMap API v0.6, `Geo/raw/*.json` | ODbL 1.0 | verified |
-| Elevation data — USGS 3DEP 1 m DEM | `epqs.nationalmap.gov` point service sampled; tile not yet downloaded | US Government work, public domain | verified as a source; acquisition and manifest pending |
+| Elevation data — USGS 3DEP 1 m DEM | `elevation.nationalmap.gov` 3DEP ImageServer export, `Geo/raw/dem-3dep-1m-26911-2026-11-06.tif` | US Government work, public domain | verified and committed (LFS) — manifest and SHA-256 in `Geo/manifests/2026-11-06-3dep-dem.md` |
 
 Attribution string for any OSM-derived output: "© OpenStreetMap contributors", <https://www.openstreetmap.org/copyright>.