qa: walkable meshes use SIMPLE_AND_COMPLEX collision (WIP on fall-through)

- ue_import_site.py: switch walkable surfaces (terrain, road, sidewalks) from
  CTF_USE_COMPLEX_AS_SIMPLE to CTF_USE_SIMPLE_AND_COMPLEX so the capsule
  blocks against the auto-generated simple collision as well as triangles.
- Re-imported site meshes, rebuilt DemoMap with the PlayerStart cap at Z300.

Unresolved: packaged plain launch still falls through at spawn in the last
verified run (capsule Z-123 -> -1960, no ground trace hit). User confirms the
site geometry RENDERS (visible in the distance) but the player is not standing
on it. Needs an in-editor/in-window collision verification — headless traces
are unreliable. Continue on this branch; do not merge to main.

Tools/ue_import_site.py

@@ -77,8 +77,12 @@ def import_fbx(path, name, collision="complex"):
         log(f"{name:18} import ok  size=({mx.x - mn.x:.0f}, {mx.y - mn.y:.0f}, {mx.z - mn.z:.0f}) cm  "
             f"min_z={mn.z:.1f}  collision=none")
         return
-    trace = (unreal.CollisionTraceFlag.CTF_USE_COMPLEX_AS_SIMPLE
-             if collision == "complex" else unreal.CollisionTraceFlag.CTF_USE_SIMPLE_AND_COMPLEX)
+    # Walkable surfaces block with SIMPLE + COMPLEX collision so the player's
+    # capsule always has ground: CFG_USE_SIMPLE_AND_COMPLEX uses the auto-
+    # generated simple collision AND the mesh triangles. (Complex-as-simple
+    # alone only worked if the mesh carried complex collision, which a default
+    # FBX static-mesh import does not.)
+    trace = unreal.CollisionTraceFlag.CTF_USE_SIMPLE_AND_COMPLEX
     try:
         body = mesh.get_editor_property("body_setup")
         body.set_editor_property("collision_trace_flag", trace)