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_import_retail_detail.py

@@ -0,0 +1,94 @@
+# Import the retail frontage trim (6800 Lee Drug, 6806–6820) into Unreal.
+#
+#   blender -b --factory-startup --python Tools/blender/retail_frontage_build.py -- \
+#       --out /tmp/gigline/retail --report /tmp/gigline/retail-report.json
+#   <engine>/Engine/Binaries/Linux/UnrealEditor-Cmd <uproject> -run=pythonscript \
+#     -script=<repo>/Tools/unreal/ue_import_retail_detail.py -nullrhi -unattended -nopause -stdout
+#
+# Two meshes arrive, one per building, each already at its world position in the
+# project frame (the FBX carries the offset), so staging spawns at the origin.
+# Destination: /Game/Hollywood/Frontage/<name> — same folder as the block frontage
+# so the demo builder stages them together.
+#
+# Result file: /tmp/gigline/retail-import.txt
+
+import glob
+import os
+
+import unreal
+
+SRC = "/tmp/gigline/retail"
+DEST = "/Game/Hollywood/Frontage"
+RESULT = "/tmp/gigline/retail-import.txt"
+lines = []
+
+
+def log(msg):
+    lines.append(str(msg))
+    unreal.log(str(msg))
+
+
+def import_fbx(path, name):
+    task = unreal.AssetImportTask()
+    task.filename = path
+    task.destination_path = DEST
+    task.destination_name = name
+    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 = True
+    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])
+
+    mesh = unreal.EditorAssetLibrary.load_asset(f"{DEST}/{name}")
+    if mesh is None:
+        log(f"MISSING after import: {DEST}/{name}")
+        return None
+
+    # Trim is decoration on a solid the player already collides with: no collision
+    # of its own, otherwise the cornice and mullions become snag points.
+    try:
+        body = mesh.get_editor_property("body_setup")
+        body.set_editor_property("collision_trace_flag",
+                                 unreal.CollisionTraceFlag.CTF_USE_COMPLEX_AS_SIMPLE)
+        mesh.set_editor_property("body_setup", body)
+    except Exception as exc:  # noqa: BLE001
+        log(f"{name}: collision flag not set ({exc!r})")
+
+    box = mesh.get_bounding_box()
+    mn, mx = box.min, box.max
+    log(f"{name:22} X {mn.x:8.0f}..{mx.x:8.0f}  Y {mn.y:7.0f}..{mx.y:7.0f}  "
+        f"Z {mn.z:5.0f}..{mx.z:5.0f}  slots {len(mesh.get_editor_property('static_materials'))}")
+    return mesh
+
+
+def main():
+    files = sorted(glob.glob(os.path.join(SRC, "*.fbx")))
+    if not files:
+        log(f"no FBX in {SRC}; run Tools/blender/retail_frontage_build.py first")
+        return
+    for path in files:
+        import_fbx(path, os.path.splitext(os.path.basename(path))[0])
+    unreal.EditorLoadingAndSavingUtils.save_dirty_packages(True, True)
+
+
+try:
+    main()
+except Exception as exc:  # noqa: BLE001
+    import traceback
+    log(f"FAILED: {type(exc).__name__}: {exc}")
+    log(traceback.format_exc())
+
+with open(RESULT, "w") as f:
+    f.write("\n".join(lines) + "\n")