Raise one signing per document for one set of parties

Batch operations group documents for coordinated creation and status tracking, with one API call and one batch identifier. Each document follows its own signing ceremony and receives its own signature. Signers complete the authentication and signing steps for each document.

Creation is atomic: either every signing in the batch is raised, or the error response means none was. Combined source bytes and combined prepared bytes each have a 100,000,000-byte limit, including repeated uses of the same file token. Known oversized inputs are refused before stored-file reads; remote downloads and stored-file reads use the remaining byte allowance. Aggregate overflow returns 413. A remote-download refusal returns 400 with the document URL pointer. Flattening complexity and its cooperative processing budget are shared across all batch documents.