feat: headless Blosm blockout of the demo block

Tools/blosm_blockout.py imports the committed extract in 3Dsimple mode and
reports per-building dimensions so a units failure is visible in the log. The
result (87 meshes) is committed as the blockout baseline: hero 25.68 x 54.88 x
14.50 m against 25.75 x 54.88 m of OSM geometry, so the import is 1:1 metres.

Two headless defects needed working around: the asset-directory check that
fires before mode is honoured, and an empty default-level table sending the
add-on down a broken relative import. Member fetching stays off so the import
does not depend on Overpass.

Docs/setup.md

@@ -47,7 +47,7 @@ The archive on disk is **Unreal Engine 5.2.0**, released May 2023, three and a h
 
 - `[x]` Blender 5.2 LTS (Flatpak) and MPFB 2.0.18 recorded.
 - `[x]` Install QGIS and record its version. QGIS **4.2.3**, Flatpak `stable`, runtime `org.kde.Platform 6.11`, bundled PROJ `9.9.0`. The stale `lts` 3.28.9 install is still beside it and should be uninstalled.
-- `[ ]` Install Blosm in Blender, record its version, and confirm it loads on Blender 5.2. Fall back to a side-by-side Blender 4.5 LTS only for the blockout step if it does not.
+- `[x]` Install Blosm in Blender, record its version, and confirm it loads on Blender 5.2. Blosm **2.7.28** imports the block extract headless in `3Dsimple` mode; the exact command and the two required headless workarounds are in `Art/blender/README.md`. No second Blender version was needed.
 - `[ ]` Confirm the geo tool Python environment (venv with `pyproj`, `GDAL`, `shapely` from the QGIS or distrobox environment) and record it.
 
 ## 4. Project bootstrap