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.

.gitignore

@@ -43,3 +43,6 @@ Thumbs.db
 
 # Local, private reference material — never committed (see Docs/reference-notes-hollywood.md)
 /Art/reference/
+
+# Blender/Blosm working caches
+/Art/blender/.cache/