Implement continuity handoffs and pickup validation

internal/store/store.go

@@ -209,6 +209,21 @@ func (s *Store) PutArtifact(b []byte) (string, error) {
 	return h, nil
 }
 
+// Artifact returns a CAS artifact after verifying its content address.
+func (s *Store) Artifact(ref string) ([]byte, error) {
+	if len(ref) != 64 {
+		return nil, fmt.Errorf("%w: invalid artifact reference", domain.ErrInvalid)
+	}
+	b, err := os.ReadFile(filepath.Join(s.cas, ref))
+	if err != nil {
+		return nil, err
+	}
+	if domain.Hash(b) != ref {
+		return nil, fmt.Errorf("%w: corrupt artifact", domain.ErrInvalid)
+	}
+	return b, nil
+}
+
 func (s *Store) Task(id string) (domain.Task, bool) {
 	s.mu.Lock()
 	defer s.mu.Unlock()