Consumes two multipart/form-data fields:

1. metadata â€" JSON body SignStreamRequest
   
2. file â€" binary data to sign
   
   
   Produces a detached-JWS in JSON format streamed as application/octet-stream.
   Output structure depends on key type:
   
   • SIMPLE keys: Single signature in compact detached format
   
   • COMPOSITE keys: Multiple signatures (classical + PQC) with anka_composite_kid field