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.

ROADMAP.md

@@ -27,8 +27,8 @@ Status detail in [Docs/setup.md](Docs/setup.md); workstation audit in [Docs/envi
 ### Geographic data
 
 - [x] Save a versioned OSM extract; document date, bounds, and source in `Geo/`.
-- [-] Acquire separately licensed elevation data if needed. Needed: measured relief is 3.94 m over the block (`Geo/verification/2026-11-06-elevation.md`). Source chosen: USGS 3DEP 1 m DEM, public domain, vertical datum NAVD88.
-- [ ] Download the 3DEP 1 m DEM for the demo bounds, reproject it to EPSG:26911 with the recorded origin and vertical datum, and record the source, clip bounds, and checksum in `Geo/`.
+- [x] Acquire separately licensed elevation data if needed. Needed and acquired: USGS 3DEP 1 m DEM in EPSG:26911, public domain, NAVD88, 984 KB committed (`Geo/manifests/2026-11-06-3dep-dem.md`); measured relief 3.94 m.
+- [x] Download the 3DEP 1 m DEM for the demo bounds, reproject it to EPSG:26911 with the recorded origin and vertical datum, and record the source, clip bounds, and checksum in `Geo/`.
 - [x] In QGIS, set a suitable local projected CRS (e.g. EPSG:26911) and record a local origin. EPSG:26911, origin and projected easting/northing are recorded in `Geo/manifests/local-origin.md` with PROJ 9.9.0 control points; QGIS 4.2.3 is installed for the DEM reprojection step.
 - [x] Verify the address, building footprints, and demo boundaries for 6840 Hollywood Blvd against source data.
 - [ ] Import OSM blockouts through Blosm; clean and detail them in Blender.