GuardFailure

The result of a failed guard evaluation.

Sum-aware: carries structural information about which guards failed and why. Unauthorized is intentionally a distinct case from Leaf so that callers (e.g. MEDECA-377, MEDECA-378) can distinguish "missing data" from "missing permission" without further refactoring.

• Leaf — a data guard (Requirement) failed; carries the predicate label and message key
• Unauthorized — an auth guard (RequireRole or RequireIdentity) failed
• Conjunction — an And guard: all failing branches accumulated (flat, no nesting)
• Disjunction — an Or guard: all branches failed (node-level, two elements)