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.

Tools/render_screenshots.sh

@@ -20,18 +20,29 @@ PROJ="$REPO/Game/GigLine/GigLine.uproject"
 VERIFY="no"
 [ "${1:-}" = "--verify" ] && VERIFY="yes"
 
-echo "== 1/3 stage the demo shot cameras =="
-cd "$ENGINE"
-./Engine/Binaries/Linux/UnrealEditor-Cmd "$PROJ" \
-  -run=pythonscript -script="$REPO/Tools/ue_build_review_map.py" \
-  -nullrhi -unattended -nopause -stdout
+run_cmdlet() {
+  local script="$1" label="$2"
+  echo "== $label =="
+  ( cd "$ENGINE" && ./Engine/Binaries/Linux/UnrealEditor-Cmd "$PROJ" \
+      -run=pythonscript -script="$REPO/Tools/$script" \
+      -nullrhi -unattended -nopause -stdout )
+}
+
+# Order matters. The demo builder 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 ended up untextured once.
+run_cmdlet ue_build_demo_map.py "1/5 rebuild the demo map (materials, site, frontage)"
+run_cmdlet ue_apply_facade_kit.py "2/5 assemble the facade kit"
+run_cmdlet ue_stage_theatre_trim.py "3/5 stage the theatre trim"
+run_cmdlet ue_audit_scene.py "4/6 audit the scene (materials, placement, labels)"
+run_cmdlet ue_build_review_map.py "5/6 stage the demo shot cameras"
 tail -3 /tmp/ue-review-map-result.txt 2>/dev/null || true
 
-echo "== 2/3 capture the demo map =="
+echo "== 6/6 capture the demo map =="
 ./Engine/Binaries/Linux/UnrealEditor-Cmd "$PROJ" /Game/Hollywood/DemoMap \
   -game -windowed -ResX=1920 -ResY=1080 -nopause -nosplash -stdout
 
-echo "== 3/3 collect the demo screenshots =="
+echo "== collect the demo screenshots =="
 mkdir -p "$REPO/Screenshots/demo"
 rm -f "$REPO"/Screenshots/demo/review-*.png
 found=0
@@ -47,6 +58,14 @@ done
 [ "$found" = "1" ] || { echo "ERROR: no demo screenshots found"; exit 1; }
 ls -1 "$REPO/Screenshots/demo"
 
+# The scene audit is a gate: a wrong-material or misplaced actor is exactly the
+# kind of failure that still renders plausibly.
+if grep -q "failure(s) ==" /tmp/gigline/scene-audit.txt 2>/dev/null; then
+  grep -E "FAIL|failure\(s\)" /tmp/gigline/scene-audit.txt | tail -5
+  grep -q "== 0 failure(s) ==" /tmp/gigline/scene-audit.txt || {
+    echo "ERROR: scene audit failed — see /tmp/gigline/scene-audit.txt"; exit 1; }
+fi
+
 if [ "$VERIFY" = "yes" ]; then
   echo "== verify: does each shot show what it should? =="
   cd "$REPO"