refactor(tools): group the sixty scripts by what they talk to

One directory with sixty scripts stopped being navigable, and the grouping is also the sandbox boundary:

  Tools/unreal/    commandlets and the two map builders, with ue_common.py and ue_placement.py as the
                   shared imports (they need the engine)
  Tools/blender/   builders, exporters, the FBX preset, the MPFB character pipeline, blender_screenshots
                   (they need the Blender Flatpak)
  Tools/review/    the visual loop: render_screenshots.sh, judge_screenshot.py, hero_compare.py,
                   build_comparison_md.py, e2e_asset_check.py, hero_iterate.sh, visual_loop.py,
                   visual_targets.json
  Tools/gen/       image and texture generation: the ComfyUI driver, deterministic texture writers,
                   reference fetching
  Tools/input/     OS-level input injection
  root             _common.py, check-lfs.sh, addon_compat_probe.py, sibling_asset_audit.py, tests/

Every moved script now resolves the repository root as ../.. and puts the tools root plus its siblings on
sys.path, so 'import ue_common' still works from unreal/. The test suite bootstrap does the same for the five
folders. render_screenshots.sh takes the folder as a third argument to run_cmdlet, and the e2e driver,
hero_iterate.sh and the comparison generator were repointed.

51 README rows now link to their new location, the README gains a Layout section naming each folder's
sandbox, and the architecture notes and Blender README follow the moves.

Verified: 108 tests pass, prettier is clean, judge_screenshot.py and hero_compare.py run from their new
homes, and the e2e run was aborted mid-way (it had been rebuilding an empty DemoMap for 15 minutes) and is
being rerun against the new layout.

Tools/unreal/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/unreal/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()