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/vehicle_from_template.py

@@ -67,6 +67,16 @@ def classify(obj):
     return "body"
 
 
+def min_z(objects):
+    from mathutils import Vector
+    lowest = None
+    for obj in objects:
+        for corner in obj.bound_box:
+            z = (obj.matrix_world @ Vector(corner)).z
+            lowest = z if lowest is None else min(lowest, z)
+    return lowest if lowest is not None else 0.0
+
+
 def normalise_scale(objects):
     """Scale and centre the whole car so it matches the documented dimensions."""
     xs, ys, zs = [], [], []
@@ -89,7 +99,9 @@ def normalise_scale(objects):
         obj.location = tuple(l * factor for l in obj.location)
     bpy.context.view_layer.update()
 
-    # Drop the car onto Z 0 and centre it on the origin in X/Y.
+    # Drop the car onto Z 0 and centre it on the origin in X/Y. Ground on the WHEELS, not
+    # on the whole model: the lowest geometry in the file is exhaust/underbody, so grounding
+    # on everything left the tyres floating by the difference.
     xs, ys, zs = [], [], []
     for obj in objects:
         for corner in obj.bound_box:
@@ -97,8 +109,11 @@ def normalise_scale(objects):
             xs.append(world.x)
             ys.append(world.y)
             zs.append(world.z)
+    wheels = [o for o in objects if "rubber" in [m.name.lower() for m in o.data.materials if m]]
+    ground = min_z(wheels) if wheels else min(zs)
+    log(f"grounding on {'wheels' if wheels else 'whole model'}: lowest Z {ground:.3f} m")
     offset = __import__("mathutils").Vector(
-        (-(min(xs) + max(xs)) / 2.0, -(min(ys) + max(ys)) / 2.0, -min(zs)))
+        (-(min(xs) + max(xs)) / 2.0, -(min(ys) + max(ys)) / 2.0, -ground))
     for obj in objects:
         obj.location = tuple(l + o for l, o in zip(obj.location, offset))
     bpy.context.view_layer.update()