I propose that humans use Unicode U+2E3B three em dash ⸻ it is an impressively long character.

let’s market it as “human dash”

And if it ever catches on with LLMs ⸻⸻ we just make it longer

Just write —(human)— to denote that a human wrote the dash. Just be sure to instruct your LLM to write —(LLM)— so readers know the difference.