Function validate_restore 

pub fn validate_restore(spec: &RestoreSpec) -> ValidationResult

A Restore spec is internally consistent (ADR §3.6/§4.6).

The externally-tagged RestoreSource/RestoreTarget enums already guarantee exactly one variant — that is a compile-time/serde invariant, not re-checked here. We validate the cross-field rules the enums can’t express:

• source.identity requires spec.repository (nothing else can derive it).
• if target: pvc, the template must name the PVC (name non-empty).