Function branch_put 

pub fn branch_put(
    db: &Db,
    branch: &str,
    coll: &str,
    id: &str,
    data: Value,
) -> Result<BranchWrite, Error>

Write a document on a branch. Invisible to the destination until merge.

PHASE 5B: becomes child_store.put(coll, id, data) against the branch’s own store and sequence counter. The signature and the isolation guarantee do not change; what changes is that the write stops consuming a parent sequence.