Function serialise_internal_node 

pub fn serialise_internal_node(
    maximum_size: usize,
    key_width: usize,
    length: usize,
    search_prefix: &[u8],
    items: &[InternalNodeChild],
) -> Result<Vec<Vec<u8>>, Error>

Build the byte chunks that [InternalNode.serialise] would emit before adding them to a store. items must be sorted by prefix (the Python loop does sorted(self._items.items())).

length is the InternalNode’s _len — the total number of leaf entries reachable through this node, not items.len() (which is the direct fan-out count).

Mirrors InternalNode.serialise minus the I/O side and the recursive walk that flushes child nodes first.