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_site.py

@@ -0,0 +1,127 @@
+# Import the Blender site meshes (terrain, road, sidewalks, kerbs) into Unreal.
+#
+#   blender ... --python Art/blender/site/export_site_fbx.py -- --out /tmp/gigline/site
+#   <engine>/.../UnrealEditor-Cmd <uproject> -run=pythonscript \
+#     -script=<repo>/Tools/unreal/ue_import_site.py -nullrhi -unattended -nopause -stdout
+#
+# Each mesh is a separate FBX because they carry different materials and want
+# different collision settings (terrain and street are walkable surfaces).
+# Destination: /Game/Hollywood/Site/<name>.
+#
+# Y-axis note: the FBX preset maps Blender +Y (north) to Unreal -Y, so the site
+# arrives in the same frame as the hero facade. Nothing is mirrored here.
+
+import glob
+import os
+
+import unreal
+
+SRC = "/tmp/gigline/site"
+DEST = "/Game/Hollywood/Site"
+RESULT = "/tmp/gigline/site-import.txt"
+lines = []
+
+
+def log(msg):
+    lines.append(str(msg))
+    unreal.log(str(msg))
+
+
+def import_fbx(path, name, collision="complex"):
+    # 5.8 note: there is no FBX import 'no collision' switch exposed to Python;
+    # a default import simply generates no collision primitives, so collision="none"
+    # means: skip the trace-flag step below and leave the body setup empty.
+    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
+    # Generate collision at import. A default FBX static-mesh import creates NO
+    # collision primitives, so CTF_USE_COMPLEX_AS_SIMPLE later has nothing to use
+    # and the player falls through the walkable surfaces (observed at runtime:
+    # ground trace under the spawn hits nothing). auto_generate_collision bakes
+    # collision geometry into the mesh so the walkable plane actually blocks.
+    smd.auto_generate_collision = True
+    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
+    # Collision is requested at import time so the 0.1.0 walkability check has
+    # something to stand on. Terrain and street get complex-as-simple: a simple
+    # hull would ramp straight over the kerbs.
+    #
+    # UE 5.8 exposes this only through the body setup's trace flag (the mesh itself
+    # has no trace-flag property, and the subsystem has no setter in Python), so
+    # the import path sets the body setup's trace flag, then verifies it by reading
+    # it back.
+    if collision == "none":
+        box = mesh.get_bounding_box()
+        mn, mx = box.min, box.max
+        log(f"{name:18} import ok  size=({mx.x - mn.x:.0f}, {mx.y - mn.y:.0f}, {mx.z - mn.z:.0f}) cm  "
+            f"min_z={mn.z:.1f}  collision=none")
+        return
+    # Walkable surfaces block with SIMPLE + COMPLEX collision so the player's
+    # capsule always has ground: CFG_USE_SIMPLE_AND_COMPLEX uses the auto-
+    # generated simple collision AND the mesh triangles. (Complex-as-simple
+    # alone only worked if the mesh carried complex collision, which a default
+    # FBX static-mesh import does not.)
+    trace = unreal.CollisionTraceFlag.CTF_USE_SIMPLE_AND_COMPLEX
+    try:
+        body = mesh.get_editor_property("body_setup")
+        body.set_editor_property("collision_trace_flag", trace)
+        mesh.set_editor_property("body_setup", body)
+    except Exception as exc:  # noqa: BLE001 - report, do not fail the whole import
+        log(f"{name}: could not set trace flag ({exc}); leaving the default")
+    box = mesh.get_bounding_box()
+    mn, mx = box.min, box.max
+    log(f"{name:18} import ok  size=({mx.x - mn.x:.0f}, {mx.y - mn.y:.0f}, {mx.z - mn.z:.0f}) cm  "
+        f"min_z={mn.z:.1f}  collision={collision}")
+
+
+def main():
+    files = sorted(glob.glob(os.path.join(SRC, "*.fbx")))
+    if not files:
+        log(f"no FBX in {SRC}; run Art/blender/site/export_site_fbx.py first")
+        return
+    for path in files:
+        stem = os.path.splitext(os.path.basename(path))[0]
+        # Crossing markings are paint on the road surface — no collision at all,
+        # so the player never stands on a 1 cm lip at the band edges.
+        if stem == "Crossing_Markings":
+            import_fbx(path, stem, "none")
+            continue
+        # Segmented ground surfaces keep their names with a numeric suffix; both the base
+        # names and the slices want the same walkable collision.
+        walkable = ("Terrain_DEM", "Road_Hollywood", "Road_Hollywood_",
+                    "Sidewalk_South", "Sidewalk_North", "Kerb_South", "Kerb_North")
+        collision = "complex" if any(stem.startswith(w) for w in walkable) else "simple"
+        import_fbx(path, stem, collision)
+    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")