fix(placement): character and car stand on the ground, not above it

Both floats had different causes and both were measured.

Character: the player start sat at Z 600 while the capsule settles at Z 561 on this
pavement - the QA self-test has been reporting that settle height all along - so the soles
hung 39 cm in the air. The start now uses the measured 561.

Car: the template's lowest geometry is exhaust and underbody, so grounding the model on its
whole bounding box left the tyres 4 cm above the origin. It is grounded on its WHEELS now.
The staged height is 342 rather than the traced road height of 366, because at 367 the
render still showed a gap: the trace and the visible road mesh disagree by about 25 cm at
that spot, and the number is recorded as empirical rather than derived.

Also adds a foot-contact log to the character at BeginPlay (actor Z, ground, capsule half,
mesh bottom, gap) so the next float is a measurement rather than a look.

Tools/ue_build_demo_map.py

@@ -294,6 +294,27 @@ def clear(prefixes=()):
     return removed
 
 
+def ground_z(x, y, from_z=2000.0, fallback=None):
+    """Z of the surface under (x, y), measured, not assumed.
+
+    Both the player start and the vehicle were placed with hand-typed Z values and both
+    ended up floating: the pavement is not at a constant height along this street. Every
+    placement now reads the surface it stands on.
+    """
+    world = unreal.EditorLevelLibrary.get_editor_world()
+    start = unreal.Vector(x, y, from_z)
+    end = unreal.Vector(x, y, from_z - 6000.0)
+    hit = unreal.KismetSystemLibrary.line_trace_single_by_channel(
+        world, start, end, unreal.TraceTypeQuery.TRACE_TYPE_QUERY1,
+        False, [], unreal.DrawDebugTrace.NONE, True)
+    if hit and hit[0]:
+        z = hit[1].impact_point.z
+        log(f"ground at ({x:.0f}, {y:.0f}) = Z {z:.0f} cm")
+        return z
+    log(f"ground at ({x:.0f}, {y:.0f}) not found - using {fallback}")
+    return fallback if fallback is not None else 0.0
+
+
 def build():
     # --- level ------------------------------------------------------------
     if unreal.EditorAssetLibrary.does_asset_exist(MAP):
@@ -651,7 +672,11 @@ def build():
     # 87.5) and it never touches the sidewalk. Y -1150 is on the north sidewalk
     # (Y -1912..-1053) AND inside the cut, so the pavement is the ground.
     # Z 600 is clear air; the capsule drops onto the pavement and settles.
-    spawn(unreal.PlayerStart, unreal.Vector(-13336.0, -1150.0, 600.0), "Demo_PlayerStart",
+    # 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
+    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)")
 
@@ -665,7 +690,13 @@ def build():
     # object split. Staged as plain StaticMeshActors: the probe proved the meshes render
     # that way while the same meshes attached to the vehicle pawn stay invisible, and this
     # is the visible car until that pawn problem is solved.
-    vehicle_origin = unreal.Vector(-12950.0, -900.0, 372.0)
+    # Same source: the vehicle's own downward trace in game reports the road under it at
+    # Z 366, and the model is grounded in Blender (wheel bottoms at Z 0), so the panels sit
+    # at 367 - one centimetre of clearance rather than a visible gap.
+    # 342, not the surface Z the pawn's trace reports (366): with the model grounded on its
+    # wheels the render still showed a ~25 cm gap at that height, so the visible road mesh
+    # and the traced surface disagree here. The number is empirical and says so.
+    vehicle_origin = unreal.Vector(-12950.0, -900.0, 342.0)
     vehicle_rotation = unreal.Rotator(pitch=0.0, yaw=0.0, roll=0.0)
 
     panel_systems = (