server.py builds an opaque dict at submission time from extra_data and pins
it on PromptServer.active_prompt_metadata while main.py's worker drives the
prompt. send_sync spreads the dict's key/value pairs onto outgoing payloads
so frames carry whatever tags the submission attached (today: workflow_id).
The mechanism is intentionally untyped — the transport layer doesn't know
what workflow_id means or treat any key specially. Adding a new propagated
field requires only a one-line addition in post_prompt; execution.py and
comfy_execution/progress.py are not touched.
execution.py changes: 0 lines.
