Optionalname/Name — human-readable signer name.
Optionalreason/Reason — why the document is being signed.
Optionaldate/M — signing time as a PDF date string, e.g. D:20260616120000Z.
Optionallocation/Location — physical or logical signing location.
Optionalcontact/ContactInfo — how to reach the signer.
Signature-dictionary metadata for GigaPdfDoc.signP12.