/// Memoizes the one full tagged content-validity walk, and the one full tagged
/// identity-constraint walk, per ``PureXML/Validation/XSDContext`` instance.
/// Each decomposed rule's `check` reads this cache (computing it on first use)
/// and filters by its own token, so registering the whole decomposed rule set
/// together still walks the tree exactly once, not once per rule. A fresh
/// `XSDContext` (built once per top-level `validate` call, see
/// `SchemaDocument.validate(_:)`) gets a fresh cache, so there is no staleness
/// risk across documents.
final class XSDFindingsCache: @unchecked Sendable {
    var contentFindings: [XSDFinding]?
    var identityFindings: [XSDFinding]?
}