bootstrap: finish project bootstrap — scale test, MPFB character, build docs

- Static-mesh scale test passes: 1 m = 100.00 cm, origin preserved, axis
  mapping east→+X / north→−Y / up→+Z (named presets ue_metric_v1 both sides).
- MPFB 2.0.18 acceptance passes: generated human + standard 137-bone rig
  with weights + v2 skin baked to 1024² diffuse; imports as SkeletalMesh
  with Skeleton, Material, and Texture2D in /Game/Test/Character.
- MPFB headless requires XDG_CONFIG_HOME at the Flatpak config tree;
  documented in Art/blender/README.md, probe in Tools/addon_compat_probe.py.
- Docs/build.md completed: fresh-checkout build, packaging, headless checks.
- Docs/setup.md reconciled with reality; ROADMAP/bootstrap closed out.

Art/blender/README.md

@@ -19,6 +19,7 @@ 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="$B/blender/lib" \
+XDG_CONFIG_HOME="$HOME/.var/app/org.blender.Blender/config" \
 BLENDER_USER_SCRIPTS="$C/scripts" \
 "$B/blender/blender" -b --factory-startup \
   --python Tools/blosm_blockout.py -- \
@@ -143,3 +144,17 @@ The extract is complete for the block but not for every relation inside its bbox
 - 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`.
+
+## Scale test and the Unreal frame (2026-11-07)
+
+`Art/blender/scaletest/scale-test-2026-11-07.blend` — three 1 m cubes at origin, +10 m east, +10 m north, plus one red test material. Regenerate with `Tools/scale_test_export.py`; import with `Tools/ue_scale_test_import.py`. Both are the named-preset templates for production assets.
+
+Export preset **`ue_metric_v1`** (Blender FBX): `apply_unit_scale`, `apply_scale_options='FBX_SCALE_NONE'`, `axis_forward='-Y'`, `axis_up='Z'`, `use_space_transform`, no bake-space-transform. Import preset **`ue_metric_v1`** (UE `FbxImportUI`): static mesh, import materials, uniform scale 1.0, no unit conversion, no force-front-X.
+
+Measured result in Unreal centimetres: 1 m cube → exactly 100.00 cm; origin preserved. Axis mapping **east → +X, north → −Y, up → +Z**. The north mirror is inherent to the right-handed Blender → left-handed UE conversion; the FBX importer fixes triangle winding, so geometry is not visually mirrored. Compass conversions in game code must use north = −Y.
+
+## Add-on compatibility (re-checked 2026-11-07)
+
+`Tools/addon_compat_probe.py` loads both pinned add-ons headless and runs an MPFB smoke test. Result on Blender 5.2.0 LTS: Blosm 2.7.28 imports; MPFB 2.0.18 enables and `mpfb.create_human` generates a default human (19 158 verts, UVs, no materials, no armature — skeleton and baked materials are separate steps).
+
+**Required invocation change:** every direct-binary headless run must now set `XDG_CONFIG_HOME="$HOME/.var/app/org.blender.Blender/config"`. Without it Blender resolves the extensions dir to `~/.config/blender/5.2/extensions`, where nothing is installed, and `bl_ext.blender_org.mpfb` is not discovered. Blosm is unaffected (it lives in `scripts/addons`, found via `BLENDER_USER_SCRIPTS`), but set the variable in all runs for consistency.