Status: IN
Internal nodes have no sibling pointer field; their binary layout is `[type:1B][num_keys:2B][child₀:4B]([keylen:2B][key][child:4B])...` with no right-link, and `_deserialize_internal` returns a 2-tuple `(keys, children)` vs. the leaf's 3-tuple `(keys, values, next_sibling)`.