Function issue_settled 

pub fn issue_settled(
    checkpoint: PicPcaPayload,
    realm: &dyn ArtifactSigner,
    ctx: &SettlementContext,
) -> Result<SettledIssue, ContinuityError>

Signs a checkpoint into the settled artifact chain: PIC PCA COSE → settled PIC Continuity COSE (transitions = null) → settled PIC Token JWT. This is the initialization path (checkpoint 0, e.g. after an OAuth-to-PIC exchange) and the tail of every settlement.