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.

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.
+- `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`.
 - `commons_fetch.py` — Wikimedia Commons reference fetcher. Search or exact `File:` titles, 1600 px thumbnails, provenance `sidecar.json` per folder. Writes only into the gitignored `Art/reference/`; reference images are never committed.
 
 ## Planned helpers