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.

ROADMAP.md

@@ -9,7 +9,7 @@ Living roadmap for this project. This file is the single source of truth for cur
 Status detail in [Docs/setup.md](Docs/setup.md); workstation audit in [Docs/environment.md](Docs/environment.md); pins in [Docs/toolchain.md](Docs/toolchain.md).
 
 - [x] Update Bazzite using its supported mechanism; verify NVIDIA driver and Vulkan operation.
-- [ ] Register an Epic account and accept the Unreal license; obtain a pinned Linux engine build or build from Epic's authorized source repository.
+- [x] Register an Epic account and accept the Unreal license; obtain a pinned Linux engine build or build from Epic's authorized source repository.
 - [ ] Validate Unreal runs on Bazzite before asset production (not an officially supported distribution).
 - [-] Install the engine-required compiler/SDK, Git, Git LFS, and an editor; use Distrobox for isolated CLI dependencies where useful.
 - [-] Pin Blender, MPFB, Blosm, and QGIS versions; confirm add-on compatibility; record in `docs/toolchain.md`.
@@ -27,7 +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.
+- [-] 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/`.
 - [-] In QGIS, set a suitable local projected CRS (e.g. EPSG:26911) and record a local origin. CRS and origin point are fixed in `Geo/manifests/local-origin.md`; the EPSG:26911 easting/northing still has to be computed with PROJ/QGIS, which also confirms the transformation.
 - [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.