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.

Docs/Architecture/05_building_block_view.md

@@ -49,7 +49,7 @@ Static meshes and materials in `/Game/Hollywood/Site/`:
 | `SM_HeroFacade` | El Capitan Entertainment Centre detailed façade |
 | `SM_ElCapitanTheatre`, `SM_Budget_6822`, `SM_Hollywood_6806`, `SM_Hollywood_6902`, `SM_Hollywood_6922`, `SM_LeeDrug_6800` | South block frontage (6 buildings) |
 | `SM_Madame_Tussauds`, `SM_TCL_Chinese_Theatre` | North-side buildings (within 50 m) |
-| `SM_Retail_LeeDrug`, `SM_Retail_6806` | Retail trim over 6800 and 6806–6820, modelled from their own photograph (`Tools/retail_frontage_build.py`, staged as `RetailTrim_*`). `6822 Budget` stays a bare volume: no photograph exists. |
+| `SM_Retail_LeeDrug`, `SM_Retail_6806` | Retail trim over 6800 and 6806–6820, modelled from their own photograph (`Tools/blender/retail_frontage_build.py`, staged as `RetailTrim_*`). `6822 Budget` stays a bare volume: no photograph exists. |
 
 ### Art Pipeline