Constant EPHEMERAL_BROADCAST_MARKER 

pub const EPHEMERAL_BROADCAST_MARKER: &str = "_ephemeral_credentials";

Top-level marker stamped onto a broadcast frame when it carries ephemeral_attributes (short-lived pairing secrets deep-merged into the live copy). Broadcast consumers use it to withhold the frame from any stream that is not bearer-authenticated — the frame’s secrets must fail closed rather than ride an unauthenticated /api/events subscriber. It is stamped only on the broadcast copy (never the persisted value, which drops ephemeral_attributes via serde(skip)) and is stripped by the SSE layer before delivery, so the public event shape is unchanged.