Function run_merge_driver 

pub fn run_merge_driver(
    base: &str,
    ours: &str,
    theirs: &str,
) -> Result<MergeDriverOutput, String>

Run the three-way merge driver on vault contents (as strings).

Parses all three versions, merges, and attempts meta regeneration. Returns the merged vault and conflict list. The caller is responsible for writing the result to disk.