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.

Tools/README.md

@@ -13,6 +13,7 @@ Repeatable import/export and processing helpers. Anything that was done twice by
 ## Helpers
 
 - `check-lfs.sh` — repository guard: fails if `git-lfs` or the `lfs` filter is missing, or if an LFS-tracked file was committed as a plain blob.
+- `site_assembly.py` — assembles terrain + street + blockout into one file and raycasts the walking routes for holes and steps.
 - `street_mesh.py` — road, kerbs, and sidewalks on the DEM, in the project frame; kerbs flush at the three crossings.
 - `terrain_from_dem.py` — terrain grid from the committed DEM, with building footprints cut out.
 - `blosm_blockout.py` — headless Blosm import of the committed `.osm` extract into a `.blend`, with a per-building scale report. Exact invocation in `Art/blender/README.md`.