Function apply_tag_columns 

fn apply_tag_columns(
    ctx_req: ContextReq,
    tag_columns: &HashMap<String, HashSet<String>>,
) -> ContextReq

Retags Field columns to Tag per table (identity makes everything a Field) so the insert path adds them to the primary key. Tags are scoped by table name so a batch targeting multiple tables can’t cross-promote a same-named field. Identity-only: rebuilds under the default opt.