A 32-year-old male labourer presents to the orthopaedic clinic 6 weeks after sustaining a closed fracture of the mid-shaft humerus. Clinical examination reveals mild swelling and tenderness at the fracture site, but no gross deformity. Plain radiographs show early callus formation bridging the fracture ends, with some resorption of the fracture margins. The patient reports pain on active movement but no neurovascular deficit. Which stage of fracture healing is most likely occurring at this point?