Function decode_shuffle_flush_flags 

pub fn decode_shuffle_flush_flags(
    flags: u64,
) -> Result<(u64, bool), &'static str>

Decode one internal clustered-shuffle flush marker into (wave, sender_activity).

Errors

Rejects an untagged marker and every bit outside the internal layout. This is deliberately stricter than the general-purpose checkpoint barrier flags because shuffle-wave metadata must never be confused with source/control-plane behavior flags.