feat: assemble the site and pre-check walkability

site-2026-11-06.blend gathers terrain, street and blockout into categorised
collections: 281 objects, ~58k triangles. A downward raycast on a 1 m grid along
both sidewalks and the designated crossing finds no holes, so nothing drops the
player on either side. Two steps are flagged: 15.8 cm at the sidewalk's outer
terrain seam and 10-13 cm ramps at the crossing flush transitions, steeper than
a real 8% ramp.

Sampling is projected through PROJ: placing points with the OSM spherical
approximation put them metres off the UTM meshes and reported 91 phantom holes.

ROADMAP.md

@@ -55,7 +55,7 @@ Status detail in [Docs/setup.md](Docs/setup.md); workstation audit in [Docs/envi
 
 ### Release checks
 
-- [ ] Player can traverse both sides of the demo without falling through geometry.
+- [ ] Player can traverse both sides of the demo without falling through geometry. Blender pre-check passes: 0 raycast holes along both sidewalks and the designated crossing; worst seam step 15.8 cm at the sidewalk's outer edge (`Art/blender/README.md`). Engine traversal still to be tested.
 - [ ] Building placement, street scale, and site identity checked against references.
 - [ ] No blocking collision or packaging defects.
 - [ ] 60 FPS at 1440p on the dev workstation (Bazzite, Ryzen 7 7700X, 64 GB RAM, RTX 5070 Ti 16 GB) using a documented quality preset; record upscaling state; report performance without frame generation.