ground_pick.gd now checks _collider_is_occluder before the walkable
break: when the ray hits a body in the "occluder" group it advances
OCCLUDER_PICK_THROUGH past the hit point and continues, unconditionally
and regardless of OcclusionPolicy fade state. Non-occluder click
targeting is unchanged.
Three unit tests added to ground_pick_test.gd covering occluder ancestry
detection. Plan open questions and module doc updated.
VinPropane