fix(materials): stop orphaning references; add a scene audit gate

The blockout cleaning pass found the map was rendering with 604 of 620 mesh actors
unmaterialed - every kit piece and all theatre trim.

Cause: ensure_material() deleted and recreated each /Game/Hollywood/Site/M_* asset on
every run, so actors staged by other tools kept references to deleted assets and fell
back to the default material. Nothing errored; the map looked merely slightly flat.

- Materials are now updated IN PLACE (load, delete_all_material_expressions, rebuild
  the graph, recompile, save). Material.expressions is protected from Python, so the
  editing library's deleter is used.
- 53 duplicated kit actor labels fixed (both shop piers of a bay shared one name).
- New Tools/ue_audit_scene.py gates the scene: every mesh actor has a material, every
  verified frontage span is within +/-1 m (via ue_placement), no duplicate labels.
- render_screenshots.sh now runs the chain in the required order (demo map -> kit ->
  theatre -> audit -> cameras -> capture) and exits non-zero on an audit failure,
  because the demo builder rebuilds materials and later staging must follow it.

Scene audit: 0 failures. Packaged QA PASS. Spawn-view reference score 0.72.

Docs/build.md

@@ -86,8 +86,13 @@ The hero block and the demo level are built by a fixed sequence. Each step write
 | North import | `Tools/ue_import_north_buildings.py` | `/Game/Hollywood/North/*` |
 | Hero import | `Tools/ue_hero_facade_import.py` | `/Game/Hollywood/ElCapitan/SM_HeroFacade` |
 | Demo map | `Tools/ue_build_demo_map.py` | `/Game/Hollywood/DemoMap` |
+| Façade kit assembly | `Tools/ue_apply_facade_kit.py` | kit actors in `DemoMap` |
+| Theatre trim | `Tools/ue_stage_theatre_trim.py` | marquee + blade sign actors |
+| Scene audit | `Tools/ue_audit_scene.py` | materials, placement spans, labels (gate) |
 | Review shots | `Tools/ue_build_review_map.py` + `Tools/render_screenshots.sh` | `Screenshots/` |
 
+**Order matters.** `ue_build_demo_map.py` rebuilds the site materials, so any actor staged by another tool must be (re)staged _after_ it, or it keeps a reference to a material that no longer exists — that is how 604 actors once ended up untethered from their materials while the map only looked slightly flat. `Tools/render_screenshots.sh` runs the full chain in the right order and gates on `ue_audit_scene.py`.
+
 Every Blender exporter calls [`Tools/fbx_export_preset.py`](../Tools/fbx_export_preset.py) (`ue_metric_v1` / `ue_character_v1`) instead of repeating the FBX argument block — one place for the scale conversion, the axis mapping and `mesh_smooth_type="FACE"` (without which every Unreal import warns about missing smoothing groups and round geometry arrives faceted).
 
 Two rules that the pipeline enforces rather than documents: