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/ue_audit_scene.py

@@ -0,0 +1,114 @@
+#!/usr/bin/env python3
+"""Audit the DemoMap for staged actors that would render wrong.
+
+Checks that are cheap and catch the failures this project actually had:
+
+1. **No unmaterialed actors.** 604 actors once rendered with the default material
+   because the demo builder deleted and recreated the site materials, orphaning
+   every reference; the map looked merely "a bit flat".
+2. **Placement spans.** Each verified frontage building is probed against
+   `Tools/ue_placement.py`; a building 13 m off its footprint still renders fine,
+   which is exactly why it needs a number, not an eye.
+3. **No duplicate actor labels.** Two buildings' kit pieces sharing a label made
+   the outliner ambiguous and hid one of them.
+
+    UnrealEditor-Cmd <uproject> -run=pythonscript \
+        -script=Tools/ue_audit_scene.py -nullrhi -unattended -nopause -stdout
+
+Result file: `/tmp/gigline/scene-audit.txt`. Exit code from the commandlet is
+always 0, so **read the file** — `visual_loop.py --verify` does not depend on it.
+"""
+
+from __future__ import annotations
+
+import collections
+import os
+import sys
+
+import unreal
+
+sys.path.insert(0, os.path.dirname(os.path.abspath(__file__)))
+import ue_placement  # noqa: E402
+
+MAP = "/Game/Hollywood/DemoMap"
+RESULT = "/tmp/gigline/scene-audit.txt"
+
+lines: list[str] = []
+failures = 0
+
+
+def log(message: str) -> None:
+    lines.append(message)
+    print(message)
+
+
+def fail(message: str) -> None:
+    global failures
+    failures += 1
+    log("FAIL " + message)
+
+
+def main() -> None:
+    if not unreal.EditorLevelLibrary.load_level(MAP):
+        log(f"could not load {MAP}")
+        return
+
+    actors = unreal.EditorLevelLibrary.get_all_level_actors()
+
+    # 1. materials
+    unmaterialed = []
+    for actor in actors:
+        comp = actor.get_component_by_class(unreal.StaticMeshComponent)
+        if not comp or not comp.static_mesh:
+            continue
+        if comp.get_material(0) is None:
+            unmaterialed.append(actor.get_actor_label())
+    log(f"mesh actors: {sum(1 for a in actors if a.get_component_by_class(unreal.StaticMeshComponent))}")
+    if unmaterialed:
+        fail(f"{len(unmaterialed)} actor(s) with no material, e.g. {unmaterialed[:6]}")
+    else:
+        log("OK   every staged mesh has a material")
+
+    # 2. placement spans
+    for actor in actors:
+        label = actor.get_actor_label()
+        key = None
+        if label.startswith("Frontage_SM_"):
+            name = label[len("Frontage_SM_"):]
+            key = {"Hollywood_6922": "6922", "Hollywood_6902": "6902",
+                   "ElCapitanTheatre": "6838", "Budget_6822": "6822",
+                   "Hollywood_6806": "6806", "LeeDrug_6800": "6800"}.get(name)
+        elif label == ue_placement.HERO_ACTOR:
+            key = "hero"
+        if not key:
+            continue
+        origin, extent = actor.get_actor_bounds(False)
+        lo, hi = origin.x - extent.x, origin.x + extent.x
+        problem = ue_placement.describe_span_failure(key, lo, hi)
+        if problem:
+            fail(problem)
+        else:
+            log(f"OK   {key}: X {lo:.0f}..{hi:.0f}")
+
+    # 3. duplicate labels
+    counts = collections.Counter(a.get_actor_label() for a in actors)
+    duplicates = {k: v for k, v in counts.items() if v > 1}
+    if duplicates:
+        sample = list(duplicates.items())[:6]
+        fail(f"{len(duplicates)} duplicated actor label(s), e.g. {sample}")
+    else:
+        log("OK   no duplicate actor labels")
+
+    log(f"== {failures} failure(s) ==")
+
+
+try:
+    main()
+except Exception as exc:  # noqa: BLE001 - surface in the result file
+    import traceback
+    log(f"FAILED: {type(exc).__name__}: {exc}")
+    log(traceback.format_exc())
+
+os.makedirs(os.path.dirname(RESULT), exist_ok=True)
+with open(RESULT, "w", encoding="utf-8") as handle:
+    handle.write("\n".join(lines) + "\n")