Function arm_then_warm 

async fn arm_then_warm(
    primary: Arc<dyn Blob>,
    opener: HedgeSiblingOpener,
    slot: Arc<OnceLock<Arc<dyn Blob>>>,
    cfg: Arc<ConfigSet>,
    metrics: BlobHedgeMetrics,
)

Installs the sibling into slot once opener yields one, then keeps it warm. See HedgedBlob::new_arming for the retry contract.