Server-Sent Events (SSE) parser for streaming inference responses.

Pure and incremental: feed it raw chunk data as it arrives off the wire and it returns parsed events plus any trailing partial frame to carry forward into the next feed/2 call.

Events:

• {:data, payload} — a completed data: frame (payload verbatim, \n joined when a frame carries multiple data: lines)
• :done — the [DONE] sentinel used by OpenAI compatible providers

Comment lines (: prefixed) and non-data fields are ignored per the SSE spec. Handles \n\n and \r\n\r\n frame separators, including separators split across chunk boundaries.