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.

Tools/ue_scale_test_import.py

@@ -0,0 +1,60 @@
+# Unreal-side half of the scale test: imports the Blender scale-test FBX with
+# the named import preset "ue_metric_v1" and measures the result.
+#
+# Run headless:
+#   <engine>/Engine/Binaries/Linux/UnrealEditor-Cmd <uproject> -run=pythonscript \
+#     -script=<repo>/Tools/ue_scale_test_import.py -nullrhi -unattended -nopause -stdout
+
+import unreal
+
+FBX = "/tmp/gigline/scaletest.fbx"
+DEST = "/Game/Test/ScaleTest"
+
+
+def import_fbx():
+    task = unreal.AssetImportTask()
+    task.filename = FBX
+    task.destination_path = DEST
+    task.automated = True
+    task.save = True
+    task.replace_existing = True
+
+    ui = unreal.FbxImportUI()
+    ui.import_mesh = True
+    ui.import_materials = True
+    ui.import_textures = False
+    ui.import_as_skeletal = False
+    smd = ui.static_mesh_import_data
+    smd.import_uniform_scale = 1.0
+    smd.combine_meshes = False
+    smd.convert_scene_unit = False
+    smd.force_front_x_axis = False
+    smd.normal_import_method = unreal.FBXNormalImportMethod.FBXNIM_IMPORT_NORMALS
+    task.options = ui
+
+    unreal.AssetToolsHelpers.get_asset_tools().import_asset_tasks([task])
+    unreal.EditorLoadingAndSavingUtils.save_dirty_packages(True, True)
+
+
+def measure():
+    # Named import preset "ue_metric_v1" measures in Unreal centimetres.
+    lines = ["---- measured local-space bounds (cm) ----"]
+    for name in ("ScaleTest_A", "ScaleTest_B", "ScaleTest_C"):
+        path = f"{DEST}/{name}"
+        asset = unreal.EditorAssetLibrary.load_asset(path)
+        if asset is None:
+            lines.append(f"MISSING asset {path}")
+            continue
+        box = asset.get_bounding_box()
+        mn, mx = box.min, box.max
+        lines.append(
+            f"{name}: origin=({(mn.x + mx.x) / 2:.2f}, {(mn.y + mx.y) / 2:.2f}, {(mn.z + mx.z) / 2:.2f}) "
+            f"size=({mx.x - mn.x:.2f}, {mx.y - mn.y:.2f}, {mx.z - mn.z:.2f})"
+        )
+    lines.append(f"assets: {unreal.EditorAssetLibrary.list_assets(DEST)}")
+    with open("/tmp/ue-scale-test-result.txt", "w") as f:
+        f.write("\n".join(lines))
+
+
+import_fbx()
+measure()