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_build_demo_map.py

@@ -372,11 +372,14 @@ def build():
     # sidewalk is at Y -1912..-1053 and the SOUTH sidewalk at Y +718..+1447.
     # The hero facade is on the SOUTH side at Y +1307. Stand on the north
     # sidewalk and face south (+Y, yaw 90) across the road at it.
-    # Z 200 is clear air so the capsule settles onto the pavement instead of
-    # spawning embedded or below grade (the fall-through QA finding).
-    spawn(unreal.PlayerStart, unreal.Vector(-13336.0, -1700.0, 200.0), "Demo_PlayerStart",
+    # Z: the terrain here is ~Z 99-138. The capsule half-height is 87.5 cm, so
+    # feet must start ABOVE the surface or the point is embedded and UE rejects
+    # the PlayerStart ("NO PLAYERSTART with positive rating"), dropping the pawn
+    # at the world origin to fall through. Cap feet ~80 cm above grade: centre
+    # ~138+80+87 = 305, so Z 300 lets it drop cleanly and gravity settles it.
+    spawn(unreal.PlayerStart, unreal.Vector(-13336.0, -1700.0, 300.0), "Demo_PlayerStart",
           unreal.Rotator(pitch=0.0, yaw=90.0, roll=0.0))
-    log("staged: Demo_PlayerStart on the north sidewalk facing south (+Y)")
+    log("staged: Demo_PlayerStart on the north sidewalk (cap above grade), facing south (+Y)")
 
     # A lockable LFS .umap checks out read-only, which makes save_map silently
     # return False — chmod u+w first or the staged actors never persist.