Function build_node_with_remapped_ids 

pub fn build_node_with_remapped_ids<NMap>(
    node_id: MastNodeId,
    node: MastNode,
    source_forest: &MastForest,
    node_remapping: &NMap,
) -> Result<MastNodeBuilder, MastForestError>
where
    NMap: LookupByIdx<MastNodeId, MastNodeId>,

Builds a node builder with remapped children using the provided mapping.

This is a generic helper used by both MastForestMerger and MastForestBuilder to avoid code duplication when copying nodes between forests.