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/toolchain.md

@@ -12,9 +12,9 @@ Audited 2026-11-06 on the development workstation.
 | NVIDIA driver | `615.78.08`, open kernel module | Bazzite image | Verified: desktop session renders through the RTX 5070 Ti; Vulkan GPU0 is the RTX 5070 Ti on driver `615.78.08`, conformance `1.4.3.3`. |
 | Unreal Engine | `5.8.4` — download in progress 2026-11-06; `5.2.0` archive verified on disk as fallback | Epic download | Not extracted yet. Pin becomes 5.8.4 once the archive verifies and the editor launches on Bazzite; see [setup.md](setup.md#engine-version-decision). |
 | Unreal Linux toolchain | engine-bundled clang | Engine source tree | Not installed. Do not substitute the system compiler. |
-| Blender | `5.2` LTS (upstream patch `5.2.3`) | Flatpak `org.blender.Blender` | Installed. |
+| Blender | `5.2.0` LTS (build 2026-07-14) | Flatpak `org.blender.Blender` | Installed. Run headless from the app tree with `LD_LIBRARY_PATH` + `BLENDER_USER_SCRIPTS`; `flatpak run` is unavailable to the agent shell. |
 | MPFB | `2.0.18` (extension `mpfb`, declares Blender >= 4.2) | Blender extensions platform | Installed. Load-and-export check on 5.2 still pending. |
-| Blosm | latest release at install time | blosm.com / Gumroad | Not installed. Version must be recorded before first use. |
+| Blosm | `2.7.28` (legacy add-on, `bl_info.blender = (4, 5, 0)`, manifest min 4.2) | blosm.com | Installed and importing on Blender 5.2 in `3Dsimple` mode. Two headless workarounds are required; see `Art/blender/README.md`. Paid material asset pack lives in `~/Downloads/blosm-assets/` and is not committed. |
 | QGIS | `4.2.3`, Flatpak `stable`, runtime `org.kde.Platform 6.11`; bundled PROJ `9.9.0` | flathub | Installed 2026-11-06. The stale `lts` branch (`3.28.9`, EOL Qt5 runtime) is still installed beside it and should be removed. |
 | Git | `2.56.0` | Bazzite image | Installed. |
 | Git LFS | `3.7.1` | `rpm-ostree` layer | Installed; `git lfs install` run, lfs filter active. |
@@ -34,7 +34,7 @@ Unreal Engine 5.9 is in preview and Unreal Engine 6 has been announced as of Nov
 
 ## Open compatibility risks
 
-1. **Blosm on Blender 5.2.** Blosm versions historically lag Blender releases. Verify Blosm against the installed Blender 5.2 before the blockout step. If it fails, add a side-by-side Blender 4.5 LTS Flatpak and convert that step's output only; do not mix Blender versions within one asset.
+1. **Blosm on Blender 5.2 — worked around, not resolved.** `3Dsimple` import works headless (see the blockout). Two headless-only defects are patched around in the script rather than in the add-on: an asset-directory check that fires before `mode` is honoured, and a background-mode import path that uses a broken relative import when the default level table is empty. `3Drealistic` mode has not been exercised; it loads the paid material pack and previously failed inside the realistic renderer setup. If realistic materials are wanted, test that mode separately before depending on it.
 2. **MPFB 2.0.18 on Blender 5.2.** The extension declares Blender >= 4.2 but predates 5.x. The acceptance test is the `0.1.0` requirement to export one MPFB character with baked materials and a deform skeleton and import it into Unreal.
 3. **Two QGIS installs.** `stable` (4.2.3, current runtime) and the old `lts` (3.28.9, EOL runtime) are both present. Nothing breaks, but launching the wrong one silently gives older PROJ data. Uninstall `lts` and keep one pin. QGIS 4.x also uses Qt6, so any processing recipe copied from a 3.x tutorial needs re-checking before it is trusted.
 4. **Driver and Vulkan validation on Bazzite.** Not an officially supported Unreal distribution. Validate before asset production, as `ROADMAP.md` requires.