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.

Art/blender/README.md

@@ -0,0 +1,55 @@
+# Blender sources
+
+Editable `.blend` files. Everything here is regenerable or hand-editable, and every generated file records the exact command that produced it.
+
+## Blockout
+
+`blockout/blockout-2026-11-06.blend` — OSM blockout for the demo bounds, imported with Blosm from the committed extract. 276 objects, 87 meshes, in collections split by feature (`…_buildings`, `…_roads_secondary`, `…_paths_footway`, `…_roads_primary`, …).
+
+### Rerun command
+
+The Flatpak Blender cannot be launched with `flatpak run` from an agent shell, so the binary inside the app tree is called directly with the Flatpak user scripts path. This is exact, not approximate:
+
+```bash
+B=/var/lib/flatpak/app/org.blender.Blender/x86_64/stable/active/files
+C="$HOME/.var/app/org.blender.Blender/config/blender/5.2"
+
+LD_LIBRARY_PATH="$B/blender/lib" \
+BLENDER_USER_SCRIPTS="$C/scripts" \
+"$B/blender/blender" -b --factory-startup \
+  --python Tools/blosm_blockout.py -- \
+  --osm Geo/raw/osm-api-map-2026-11-06.osm \
+  --bounds 34.1006,-118.3422,34.1020,-118.3370 \
+  --assets-dir /run/media/system/DATA/Downloads/blosm-assets/assets \
+  --out Art/blender/blockout/blockout-2026-11-06.blend
+```
+
+Run time is about 36 s for this extent. Two headless-only workarounds are baked into the script and must stay there:
+
+1. Blosm refuses to import until an asset directory is set; the paid asset pack path is passed explicitly and is **not** committed.
+2. In background mode the GUI never fills Blosm's default level table, and the add-on then takes a code path with a broken relative import. The script seeds one entry.
+
+## Scale check (blockout, 2026-11-06)
+
+Blender works in metres, and the imported meshes match the OSM geometry, so the blockout is 1:1 before any Unreal export:
+
+| Building | Measured in Blender | OSM geometry | Height source |
+| --- | --- | --- | --- |
+| El Capitan Entertainment Centre | 25.68 × 54.88 × 14.50 m | 25.75 × 54.88 m | `height` tag 14.5 |
+| El Capitan Theatre | 38.59 × 54.87 × 20.62 m | 38.59 × 54.87 m | `building:levels=6` × 3.2 m, plus roof |
+| Madame Tussaud's | 42.41 × 57.20 × 15.90 m | 42.0 m frontage | `height` tag 15.9 |
+| TCL Chinese Theatre | 43.75 × 76.19 × 18.10 m | 44.0 m frontage | `height` tag 18.1 |
+
+The 0.07 m width difference on the hero is Blosm's own projection of the outline, not a units problem. Anything larger than a few centimetres would be a flag.
+
+Buildings **without** a `height` tag get levels × 3.2 m, which is a placeholder, not evidence. Every such height is a modelling decision to be replaced from the reference photography.
+
+## Relation completeness
+
+The extract is complete for the block but not for every relation inside its bbox: Blosm's optional member fetch pulled six ways from Overpass, all belonging to one 213 × 152 m multipart complex about 100 m north of the boulevard — outside the `0.1.0` bounds and out of scope. That fetch is therefore **off by default** (`--load-missing` enables it), so the committed blockout is reproducible offline from the committed extract alone. If background massing later needs that complex, refetch a wider extract rather than reintroducing a network dependency.
+
+## What this blockout is not
+
+- Not cleaned, not subdivided into façade modules, not UV-mapped, not detailed.
+- Materials are placeholders from Blosm's simple mode. Realistic mode needs the paid material pack and was not run.
+- Terrain, kerbs, and crossing ramps are not in it: those come from the DEM cross-sections in `Geo/verification/2026-11-06-cross-sections.md`.