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_character_anim_import.py

@@ -0,0 +1,96 @@
+# Import the character animation clips into Unreal (0.2.0).
+#
+#   blender -b Art/blender/character/character-2026-11-07.blend \
+#       --python Tools/blender/character_anim_build.py -- --out /tmp/gigline/character-anim.fbx
+#   <engine>/Engine/Binaries/Linux/UnrealEditor-Cmd <uproject> -run=pythonscript \
+#     -script=<repo>/Tools/unreal/ue_character_anim_import.py -nullrhi -unattended -nopause -stdout
+#
+# Imports the animated FBX next to the existing character assets and verifies the
+# result rather than assuming it: an FBX whose export omitted `bake_anim` imports
+# without error and produces a skeletal mesh but no AnimSequence, which is exactly
+# the failure this tool must catch.
+#
+# Destination: /Game/Test/Character. Result file: /tmp/gigline/character-anim-import.txt
+
+import glob
+import os
+
+import unreal
+
+SRC = "/tmp/gigline/character-anim.fbx"
+DEST = "/Game/Test/Character"
+PREFIX = "CharacterV6"
+RESULT = "/tmp/gigline/character-anim-import.txt"
+lines = []
+
+
+def log(msg):
+    lines.append(str(msg))
+    unreal.log(str(msg))
+
+
+def main():
+    if not os.path.exists(SRC):
+        log(f"missing {SRC}; run Tools/blender/character_anim_build.py first")
+        return
+
+    task = unreal.AssetImportTask()
+    task.filename = SRC
+    task.destination_path = DEST
+    task.destination_name = PREFIX
+    task.automated = True
+    task.save = True
+    task.replace_existing = True
+
+    ui = unreal.FbxImportUI()
+    ui.import_mesh = True
+    ui.import_as_skeletal = True
+    ui.import_animations = True
+    ui.import_materials = True
+    ui.import_textures = False
+    # UE 5.8 has no SKELETON_ONLY content type (FBXICT_ALL/GEOMETRY/SKINNING_WEIGHTS
+    # only), so the clips come in with the mesh and the verification below decides
+    # whether that worked. The mesh is replaced at the same path by design.
+    ui.skeletal_mesh_import_data.import_uniform_scale = 1.0
+    task.options = ui
+    unreal.AssetToolsHelpers.get_asset_tools().import_asset_tasks([task])
+
+    # Verify: the clips are the deliverable, not the mesh.
+    registry = unreal.AssetRegistryHelpers.get_asset_registry()
+    found = []
+    for asset in registry.get_assets_by_path(DEST, recursive=False):
+        name = str(asset.asset_name)
+        if not name.startswith(PREFIX):
+            continue
+        obj = unreal.EditorAssetLibrary.load_asset(f"{DEST}/{name}")
+        if isinstance(obj, unreal.AnimSequence):
+            length = obj.get_play_length()
+            # UE 5.8 exposes only play length and sequence length here; the sampled
+            # frame count is derived so the report still states the clip duration.
+            frames = round(length * 30.0)
+            found.append(name)
+            log(f"AnimSequence {name:34} {length:.3f} s (~{frames} frames at 30 fps)")
+        elif isinstance(obj, unreal.SkeletalMesh):
+            skeleton = obj.get_editor_property("skeleton")
+            log(f"SkeletalMesh {name:34} skeleton={'ok' if skeleton else 'MISSING'}")
+
+    if not found:
+        log("FAILED: no AnimSequence was created — the FBX carried no baked animation")
+        lines.append("VERDICT: FAIL")
+        return
+    log(f"{len(found)} AnimSequence(s) imported: {', '.join(sorted(found))}")
+    lines.append("VERDICT: PASS")
+
+
+try:
+    main()
+except Exception as exc:  # noqa: BLE001
+    import traceback
+    log(f"FAILED: {type(exc).__name__}: {exc}")
+    log(traceback.format_exc())
+    lines.append("VERDICT: FAIL")
+
+unreal.EditorLoadingAndSavingUtils.save_dirty_packages(True, True)
+os.makedirs(os.path.dirname(RESULT), exist_ok=True)
+with open(RESULT, "w", encoding="utf-8") as handle:
+    handle.write("\n".join(lines) + "\n")