feat: terrain mesh from the DEM, and correct the blockout frame

Tools/terrain_from_dem.py builds a 191x106 grid at 2 m over the block, Z being
NAVD88 minus the origin ground. Building footprints from the OSM extract are cut
out so rooftops never become terrain (72 footprints, 6310 cells). Spot checks
reproduce the measured profile: origin 0.00, Orange Dr +3.39, south gutter
+0.93, crown +1.38, north sidewalk +1.43.

Blosm projects with a sphere, not an ellipsoid, so its frame was ~0.4% oversized
and rotated by meridian convergence: about 2 m at the theatre. The blockout now
fits a six-parameter affine into EPSG:26911 on five control points (0.0 cm
residual) and transforms root objects only, since children inherit and double
transforming them read as a 2 m placement error.

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. 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`).
+- [-] Build sidewalks, roads, and collision meshes. Terrain mesh from the DEM exists (`Art/blender/terrain/terrain-2026-11-06.blend`); sidewalks, roads, kerbs and collision are not built. 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