geo: measure block elevation and pick the DEM source

Probe 22 points with the USGS 3DEP point service: walkable surfaces span
115.65-119.59 m, a 3.94 m relief, so elevation data is required by the AGENTS
rule. Grade falls ~3.3 m eastward over the 279 m block (1.2%) and the north
sidewalk sits 0.4-0.7 m above the south. 3DEP 1 m is public domain and becomes
the chosen source; the tile itself is still to be downloaded.

Factored the shared OSM helpers into Geo/scripts/osm_common.py so the street
profile and the elevation probe read the same geometry.

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 and the hero façade width remain 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 (3.94 m relief, 3DEP chosen) but the DEM tile is not downloaded; the hero façade width remains open.
 
 ## Required manifest