Create Keys Bulk

Batch key create — idempotent, best-effort per item.

Items referencing an unknown namespace (not in this project) or a (namespace, name) pair that repeats within the batch error per item; valid items still commit. An item whose key already exists returns created=false (counted as unchanged). The plan key-cap is a whole-batch gate: if the NEW keys would exceed the org cap the entire call is rejected (403) before any write.