fix(placement): give the sidewalk collision, and the player stands on it

The staged sidewalk mesh does not block a downward trace: the capsule fell straight through to
Road_Hollywood at 366 whatever height it started at, which is why three spawn values produced
identical captures. The mesh exists, carries QueryAndPhysics collision and its bounding box tops
out at Z 380, but a trace from above it - and a capsule landing on it - both pass through.

Until the kit mesh's own collision is fixed, an invisible 279 x 8.6 x 0.08 m cube supplies the
surface: top at Z 380, hidden in game, BlockAll. Verified in the same run that used to report the
road:

  top surface under player: Z=380 actor=StaticMeshActor_52 mesh=Cube
  foot Visibility: actor Z 470, feet 382, hit Z 380

So the capsule now rests on the pavement 2 cm above its surface.

The captures still read as floating in the ground-level shots, and the table records that: the
stopgap fixes what the character stands on, not what the camera sees, and those two disagree
until the sidewalk mesh's rendered top matches its collision.

Tools/ue_build_demo_map.py

@@ -675,7 +675,32 @@ def build():
     # Measured, not guessed: the QA self-test reports the capsule settling at Z 561 on this
     # pavement (capsule half height 88), so 561 puts the soles exactly on the surface. The
     # previous Z 600 started 39 cm in the air and read as floating.
-    player_z = 561.0
+    # Measured from the staged geometry: Demo_Sidewalk_North spans Y -1912..-1053 and its
+    # top face is at Z 380, and the player start's XY is inside that footprint. The capsule
+    # centre belongs at 380 + half height 88 + 1 = 469. Earlier values put the feet BELOW the
+    # pavement top, so the capsule started inside the sidewalk block and the downward trace
+    # exited through it to Road_Hollywood at 366 - which is what made the character look like
+    # it floated while standing on a surface 14 cm above the road.
+    player_z = 469.0
+    # Stopgap collision for the north sidewalk. The staged mesh (Demo_Sidewalk_North, top at
+    # Z 380) does not block a downward trace - the capsule falls straight through it to
+    # Road_Hollywood at 366 - so an invisible cube supplies the surface the player walks on.
+    # The real fix belongs in the kit mesh's collision; this keeps the demo walkable meanwhile.
+    sidewalk_collision = spawn(unreal.StaticMeshActor,
+                               unreal.Vector(-14000.0, -1482.5, 376.0),
+                               "Demo_SidewalkCollision")
+    if sidewalk_collision:
+        cube = unreal.EditorAssetLibrary.load_asset("/Engine/BasicShapes/Cube.Cube")
+        comp = sidewalk_collision.get_component_by_class(unreal.StaticMeshComponent)
+        if cube and comp:
+            comp.set_static_mesh(cube)
+            comp.set_mobility(unreal.ComponentMobility.STATIC)
+            # A 100 cm engine cube: 279 m long, 8.6 m wide, 8 cm thick -> top at Z 380.
+            sidewalk_collision.set_actor_scale3d(unreal.Vector(279.0, 8.6, 0.08))
+            comp.set_collision_profile_name("BlockAll")
+            sidewalk_collision.set_actor_hidden_in_game(True)
+            log("staged: Demo_SidewalkCollision (invisible, top at Z 380)")
+
     spawn(unreal.PlayerStart, unreal.Vector(-13336.0, -1150.0, player_z), "Demo_PlayerStart",
           unreal.Rotator(pitch=0.0, yaw=90.0, roll=0.0))
     log("staged: Demo_PlayerStart on the cut north sidewalk, facing south (+Y)")