test(placement): the character is grounded to within 2 cm, measured three ways

Ran the 158 cm attempt again, with a staging log this time, and it fails the same way: with the
slab at a 158 cm top the player falls to Z -4412 and never hits anything, while the identical
actor collides at 380. Setting the start above the slab put the player on Road_Hollywood at 366 -
so the game's ground here really is road 366 / sidewalk bbox 380, and the Blender-measured 1.58 m
does not describe the shipped map; the blend predates the terrain. The 158 hypothesis was wrong and
is retracted in TODO.md.

Geometry is now measured end to end:

  top surface under player: Z=380 mesh=Cube (actor Z 470)
  body: mesh bottom 382, capsule bottom 382, body above capsule 0 cm

The body's lowest vertex and the capsule bottom agree within 0 cm, sitting 2 cm above the collision
surface. If the captures still read as floating, the cause is the visible sidewalk mesh sitting
lower than its bounding box at that XY - and that value is not reachable from Python, so it needs
a measurement in the mesh itself.

Tools/ue_build_demo_map.py

@@ -699,7 +699,10 @@ def build():
             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)")
+            loc = sidewalk_collision.get_actor_location()
+            scale = sidewalk_collision.get_actor_scale3d()
+            log(f"staged: Demo_SidewalkCollision at Z {loc.z:.0f}, scale Z {scale.z:.3f} "
+                f"(top {loc.z + 50.0 * scale.z:.0f} cm)")
 
     spawn(unreal.PlayerStart, unreal.Vector(-13336.0, -1150.0, player_z), "Demo_PlayerStart",
           unreal.Rotator(pitch=0.0, yaw=90.0, roll=0.0))