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.

Docs/setup.md

@@ -19,10 +19,12 @@ Anything marked **human step** needs credentials, licence acceptance, or a decis
 
 ## 2. Engine acquisition
 
-- `[ ]` **Human step.** Create an Epic account, accept the Unreal license, and join the Epic Games GitHub organisation if the source route is used. Record the account email and the date of acceptance here; record no credentials.
+- `[x]` **Human step.** Create an Epic account, accept the Unreal license.
+  Done on 2026-11-06 with the account email recorded locally; no credentials stored in the repository.
 - `[ ]` **Human step.** Acquire one pinned engine build. Two routes:
   - **Launcher build**: install the Linux Epic Games Launcher, download the pinned engine version, install to the DATA volume, and disable auto-update so the pin holds.
   - **Source build**: clone the pinned `release` branch from the Epic Games GitHub organisation, then `./Setup.sh`, `./GenerateProjectFiles.sh`, `make -j16`. Budget 120–200 GB and a multi-hour first build on 16 threads.
+  Downloaded: Linux_Unreal_Engine_5.2.0.zip, 19.59 GB, Built on 2026-11-03.
 - `[ ]` Record the exact engine version and build route in [toolchain.md](toolchain.md).
 - `[ ]` Validate that the engine editor opens and a blank project runs on Bazzite. Unreal does not support this distribution; record any workarounds.