Function materialise_r2_bucket_names 

pub fn materialise_r2_bucket_names(
    text: &str,
    names: &BTreeMap<String, String>,
) -> Result<String, String>

P5 (#2323): point [[r2_buckets]] entries at their deploy-time bucket names, structurally, by binding: each entry whose binding names lists gets that bucket_name. The generated name is the unqualified derived one (r2_bucket_name); an environment qualifies it. Unchanged text when nothing differs, as materialise_kv_namespace_ids.