I put this in hoping to give people enough time to bike-shed it over the next two weeks -- I was guessing there would be at least a few messages on naming (FINAL/MSG_DONE/CONTINUE vs END_STREAM/END_MESSAGE/END_HEADERS etc.)

I chose "message boundary" because that was the wording in the websocket protocol -- I'm happy to add text to elucidate its use or say that it is reserved for future extensions to the protocol.