geo: sample seven cross-sections from the DEM

Gutters sit at -10 m south and +10..12 m north, a ~2% crown between them, and
the envelope runs 0.6-1.0 m per section. Gutter spacing independently confirms
the 20.9 m carriageway measured from the crossing. The sharpest breaks are
0.09-0.12 m/m, so 3DEP does not resolve a kerb upstand: kerb height stays a
recorded assumption rather than a derived value.

ROADMAP.md

@@ -42,7 +42,7 @@ Status detail in [Docs/setup.md](Docs/setup.md); workstation audit in [Docs/envi
 - [ ] Model hero façade for the El Capitan Entertainment Centre site.
 - [ ] Model surrounding block frontage.
 - [ ] Model simplified background buildings directly across Hollywood Boulevard.
-- [ ] Build sidewalks, roads, and collision meshes.
+- [ ] Build sidewalks, roads, and collision meshes. Ground must follow the DEM: 3.94 m fall along the block, ~2 % crown between the gutters at ±10–11 m from the centreline, north sidewalk higher than south. Kerb height is an unresolved assumption — the DEM does not resolve it (`Geo/verification/2026-11-06-cross-sections.md`).
 - [ ] Materials and look development for all exported assets.
 
 ### Gameplay