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.

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 | not yet decided | per provider | **pending** — decide after the DEM variation check in `ROADMAP.md` |
+| 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 |
 
 Attribution string for any OSM-derived output: "© OpenStreetMap contributors", <https://www.openstreetmap.org/copyright>.