Implement minimal downstream invalidation after forced upstream reruns

internal/manifest/manifest.go

@@ -122,6 +122,15 @@ func (m *Manifest) MarkStageSkipped(name string, at time.Time, reason string) {
 	m.UpdatedAt = at
 }
 
+// MarkStageStale marks a stage as stale so it is not skipped as idempotently complete.
+func (m *Manifest) MarkStageStale(name string, at time.Time, reason string) {
+	s := m.ensureStage(name, at)
+	s.Status = StatusStale
+	s.Error = &ErrorRecord{Message: strings.TrimSpace(reason), Code: "stale", At: timePtr(at)}
+	s.UpdatedAt = at
+	m.UpdatedAt = at
+}
+
 func (m *Manifest) ensureStage(name string, at time.Time) *StageRecord {
 	if m.Stages == nil {
 		m.Stages = map[string]*StageRecord{}