Function submit_fire_and_forget 

pub fn submit_fire_and_forget(
    payload: TelemetryPayload,
    endpoint: String,
) -> Sender<()>

Fire-and-forget telemetry submission.

Spawns a background thread to submit the payload. Does not block the caller. The submission result is logged but not returned.

This is the primary submission API for the main pipeline — it adds no measurable latency since it returns immediately.