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.

Art/blender/README.md

@@ -153,7 +153,7 @@ The extract is complete for the block but not for every relation inside its bbox
 
 ## Scale test and the Unreal frame (2026-11-07)
 
-`Art/blender/scaletest/scale-test-2026-11-07.blend` — three 1 m cubes at origin, +10 m east, +10 m north, plus one red test material. Regenerate with `Tools/scale_test_export.py`; import with `Tools/ue_scale_test_import.py`. Both are the named-preset templates for production assets.
+`Art/blender/scaletest/scale-test-2026-11-07.blend` — three 1 m cubes at origin, +10 m east, +10 m north, plus one red test material. Regenerate with `Tools/blender/scale_test_export.py`; import with `Tools/unreal/ue_scale_test_import.py`. Both are the named-preset templates for production assets.
 
 Export preset **`ue_metric_v1`** (Blender FBX): `apply_unit_scale`, `apply_scale_options='FBX_SCALE_NONE'`, `axis_forward='-Y'`, `axis_up='Z'`, `use_space_transform`, no bake-space-transform. Import preset **`ue_metric_v1`** (UE `FbxImportUI`): static mesh, import materials, uniform scale 1.0, no unit conversion, no force-front-X.