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.

Geo/README.md

@@ -13,7 +13,7 @@ OSM is layout data only. It is not authoritative for façade detail, height, or
 | `Geo/manifests/` | One manifest per saved extract (see below) |
 | `Geo/scripts/` | Repeatable fetch and conversion helpers |
 
-Current state: the extract, both El Capitan footprints, the demo bounds, the building sets, the street profile, and the designated crossing are recorded in [`verification/`](verification/). The local CRS and origin are fixed in [`manifests/local-origin.md`](manifests/local-origin.md), with the projected easting/northing still to be computed. Elevation is measured (3.94 m relief, 3DEP chosen) but the DEM tile is not downloaded; the hero façade width remains open.
+Current state: the extract, both El Capitan footprints, the demo bounds, the building sets, the street profile, and the designated crossing are recorded in [`verification/`](verification/). The local CRS and origin are fixed in [`manifests/local-origin.md`](manifests/local-origin.md), with the projected easting/northing still to be computed. Elevation is measured and the 3DEP 1 m DEM window is committed (EPSG:26911, NAVD88, 984 KB). The hero façade width remains open.
 
 ## Required manifest