qa wip: collide site meshes, spawn cap above grade (still investigating fall-through)

- ue_import_site.py: auto_generate_collision=True so FBX static meshes get
  real collision geometry (they imported with none -> complex-as-simple had
  nothing to use, player fell through).
- ue_build_demo_map.py: PlayerStart cap raised to Z300 so the capsule feet
  start above the ~Z99-138 terrain surface there (an embedded spawn point is
  rejected -> 'NO PLAYERSTART with positive rating' -> origin fallback).
- Re-imported site meshes + rebuilt DemoMap with the above.

KNOWN-UNRESOLVED: packaged plain launch still spawns the capsule around
Z-123 and falls; downward trace finds no geometry. Map assets are all present
in the pak and referenced; runtime collision behavior needs a visual/interactive
check (headless commandlet traces are unreliable). Investigate on this branch;
do not merge to main.

Tools/ue_import_site.py

@@ -50,6 +50,12 @@ def import_fbx(path, name, collision="complex"):
     smd.convert_scene_unit = False
     smd.force_front_x_axis = False
     smd.normal_import_method = unreal.FBXNormalImportMethod.FBXNIM_IMPORT_NORMALS
+    # Generate collision at import. A default FBX static-mesh import creates NO
+    # collision primitives, so CTF_USE_COMPLEX_AS_SIMPLE later has nothing to use
+    # and the player falls through the walkable surfaces (observed at runtime:
+    # ground trace under the spawn hits nothing). auto_generate_collision bakes
+    # collision geometry into the mesh so the walkable plane actually blocks.
+    smd.auto_generate_collision = True
     task.options = ui
     unreal.AssetToolsHelpers.get_asset_tools().import_asset_tasks([task])