Native connectors to tax authorities and e-invoicing networks

#[async_trait]
pub trait CountryConnector: Send + Sync {
    /// Schema + business-rule validation for the jurisdiction.
    async fn validate(&self, doc: &Document) -> Result<Validation, ConnectorError>;

    /// Submit to the authority / access point. Returns the official stamp.
    async fn submit(&self, doc: &Document) -> Result<Stamp, ConnectorError>;

    /// Poll the authority for the current acceptance status.
    async fn query_status(&self, id: &ExternalId) -> Result<Status, ConnectorError>;

    /// Issue a fully stamped, legally valid document, sealed into the WAL.
    async fn issue(&self, draft: Draft) -> Result<Issued, ConnectorError>;
}