Function merge_models 

pub fn merge_models(a: &ExportedModel, b: &ExportedModel) -> MergeResult

Merge two exported models into one.

Node types are unioned by name (higher-dimension embedding strategy wins on conflict). Edge types are unioned by (from, to, type) key with higher confidence kept. Causal nodes are unioned by label. Causal edges are unioned by (source, target) key with weights averaged on overlap.