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.

MEMORY.md

@@ -54,3 +54,14 @@ Scope: facts and traps that cost time to establish. Planned work lives in [ROADM
 - **Without `git lfs install` the lfs filter is inactive and `git add` does not warn** — it stores the file as a plain blob. `Tools/check-lfs.sh` is the guard.
 - **Never commit reference images or `.env`.** `.gitignore` covers `/Art/reference/`, `.env`, `*.env`. When images slipped in early, history was rewritten to remove them.
 - The project origin is Hollywood × Highland: `E 376 515.389 / N 3 774 224.083` in EPSG:26911. Blender X east, Y north, Z up, metres, Z measured from the origin ground (NAVD88 115.911 m). The whole block sits at negative X.
+
+## 2026-11-07 — Unreal pipeline findings
+
+- **UE 5.8 game targets must use `BuildSettingsVersion.V7`** to share the installed editor's build environment; V5 is rejected with "modifies the values of properties … not allowed".
+- **`UnrealEditor-Cmd -run=pythonscript` swallows Python `print()`** with `-stdout` in 5.8 — write results to a file instead (see `Tools/ue_scale_test_import.py`).
+- **`unreal.Box` has `min`/`max`, not `origin`/`get_extent()`** in 5.8 Python; compute centre/size from the corners.
+- **FBX Blender→UE axis mapping is east→+X, north→−Y, up→+Z** with preset `ue_metric_v1`; the Y mirror is inherent (RH→LH). Compass math in game code: north = −Y. Verified with the three-cube scale test.
+- Headless asset import works fine with `-nullrhi`; `unreal.AssetImportTask` + `FbxImportUI` is the pattern (Interchange pipeline under the hood).
+- **Flatpak Blender headless needs `XDG_CONFIG_HOME=$HOME/.var/app/org.blender.Blender/config`** for extensions: `bpy.utils.user_resource('EXTENSIONS')` otherwise resolves to `~/.config/blender/5.2/extensions` (empty) and `bl_ext.blender_org.mpfb` is undiscoverable. Blosm unaffected (scripts/addons via `BLENDER_USER_SCRIPTS`). Found via `Tools/addon_compat_probe.py`; documented in `Art/blender/README.md`.
+- **UE 5.8 Interchange skips material/texture assets for skeletal FBX imports** (static imports do get materials). Finish the chain in Python: import texture, build Material via `MaterialEditingLibrary`, assign with `sm.set_editor_property('materials', [SkeletalMaterial])` — the struct field is `material_interface`, not `material`, and `SkeletalMesh.set_material()` does not exist. See `Tools/ue_character_import.py`.
+- **Blender 5.2 FBX exporter removed `armature_nla`** (passes for 4.x recipes fail). Also: a baked image is only referenced by the FBX exporter when `image.filepath` (not just `filepath_raw`) is set and the exporter can map the node — MPFB's v2 skin node-group tree can't be mapped, so a flat export material is required.