Function run_observer 

pub fn run_observer(
    hook: Option<&Arc<dyn SettlementHook>>,
    receipt: &ChioReceipt,
    trusted_kernel_keys: &[PublicKey],
    idempotency_key: &SettlementIdempotencyKey,
) -> SettlementObserverStatus

Run the registered settlement hook against a freshly signed receipt.

Settlement is observer-only relative to receipt bytes: the receipt has already been signed and persisted before this function runs, and the returned status NEVER feeds back into the dispatch path. The function is plumbed through the kernel struct in super::ChioKernel::run_settlement_observer so callers only need the kernel handle.