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.

MEMORY.md

@@ -487,3 +487,24 @@ Scope: facts and traps that cost time to establish. Planned work lives in [TODO.
   twice on a clean run. Always count instances before believing a perf number.
 - Series committed at `Docs/perf/0.1.5-kit-walk-frametimes.csv`; comparison table in
   `Docs/release-checks.md`.
+
+## 2026-11-09 — 604 unmaterialed actors: material rebuild was delete+recreate
+
+- The "blockout cleaning pass" turned out to need **material and placement hygiene,
+  not more geometry**. `Tools/ue_audit_scene.py` (new) found **604 of 620 mesh actors
+  with no material**: kit pieces and theatre trim all untextured.
+- **Cause:** `ensure_material` in `ue_build_demo_map.py` deleted and recreated each
+  `/Game/Hollywood/Site/M_*` asset every run ("so a look change cannot be masked by a
+  stale asset"). That orphane every reference: actors staged by other tools pointed at
+  deleted assets and fell back to the default material. The map looked only *slightly*
+  flat — no error, no warning.
+- **Fix:** update the material **in place** — load it, `MaterialEditingLibrary.
+  delete_all_material_expressions(mat)`, rebuild the graph, recompile, save. Note
+  `Material.expressions` is protected from Python; use the library's deleter.
+- **Ordering rule now documented and enforced:** the demo builder rebuilds materials,
+  so kit and theatre staging must run *after* it. `render_screenshots.sh` now runs
+  demo map → kit assembly → theatre trim → scene audit → shot cameras → capture, and
+  **fails on a non-zero audit count**.
+- Second find: 53 duplicated kit labels (both shop piers of a bay shared one name).
+- Scene audit after the fixes: **0 failures** (all materialed, all spans verified, no
+  duplicate labels). Packaged QA still PASS; spawn-view reference score 0.72.