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/tests/test_ue_placement.py

@@ -6,6 +6,15 @@ Running the importer moved the hero 13 m east and onto the north side of the
 boulevard, and nothing complained because a displaced building still renders.
 """
 
+import os
+import sys
+
+_TOOLS = os.path.dirname(os.path.dirname(os.path.abspath(__file__)))
+sys.path.insert(0, _TOOLS)
+for _sub in ("review", "gen", "unreal", "blender", "input"):
+    sys.path.insert(0, os.path.join(_TOOLS, _sub))
+
+
 import os
 import sys
 import unittest
@@ -70,14 +79,14 @@ class SingleSourceOfTruthTest(unittest.TestCase):
     """The import script and the demo builder must not carry their own literals."""
 
     def test_import_script_uses_the_shared_constant(self):
-        src = open(os.path.join(ROOT, "Tools", "ue_hero_facade_import.py"),
+        src = open(os.path.join(ROOT, "Tools", "unreal", "ue_hero_facade_import.py"),
                    encoding="utf-8").read()
         self.assertIn("ue_placement.HERO_LOCATION_CM", src)
         self.assertNotIn("(-12024.5, -1450.0, 0.0)", src,
                          "the stale pre-0.1.0 hero position must not come back")
 
     def test_demo_builder_uses_the_shared_constant(self):
-        src = open(os.path.join(ROOT, "Tools", "ue_build_demo_map.py"),
+        src = open(os.path.join(ROOT, "Tools", "unreal", "ue_build_demo_map.py"),
                    encoding="utf-8").read()
         self.assertIn("ue_placement.HERO_LOCATION_CM", src)