Function derive_batch_id 

pub fn derive_batch_id(
    session_id: &str,
    source_key: Option<&str>,
    now_epoch_ms: u64,
    nonce: Option<u64>,
) -> String

Derive the durable id for a newly enqueued batch.

nonce disambiguates batches enqueued within the same millisecond; backends whose id uniqueness already comes from elsewhere pass None.