Implement minimal downstream invalidation after forced upstream reruns

internal/app/run_control_test.go

@@ -1,6 +1,7 @@
 package app
 
 import (
+	"reflect"
 	"testing"
 	"time"
 
@@ -40,3 +41,48 @@ func TestDecideStageActions(t *testing.T) {
 		t.Fatalf("forced prepare action = %q, want %q", forced[0].Action, stageActionRun)
 	}
 }
+
+func TestDownstreamStageNames(t *testing.T) {
+	got := downstreamStageNames("polish")
+	want := []string{"normalize", "trim", "analyze", "archive", "notify"}
+	if !reflect.DeepEqual(got, want) {
+		t.Fatalf("downstreamStageNames(polish) = %#v, want %#v", got, want)
+	}
+
+	missing := downstreamStageNames("unknown")
+	if len(missing) != 0 {
+		t.Fatalf("downstreamStageNames(unknown) = %#v, want empty", missing)
+	}
+}
+
+func TestInvalidateDownstreamSucceededStages(t *testing.T) {
+	now := time.Now().UTC()
+	m := manifest.New("2026-05-03", now)
+	m.MarkStageSucceeded("prepare", now, nil)
+	m.MarkStageSucceeded("transcribe", now, nil)
+	m.MarkStageSucceeded("merge", now, nil)
+	m.MarkStageSucceeded("polish", now, nil)
+	m.MarkStageSucceeded("normalize", now, nil)
+	m.MarkStageSucceeded("trim", now, nil)
+	m.MarkStageFailed("analyze", now, "analysis failed")
+	m.MarkStageSucceeded("archive", now, nil)
+	m.MarkStageSucceeded("notify", now, nil)
+
+	got := invalidateDownstreamSucceededStages(m, "polish", now.Add(1*time.Second))
+	want := []string{"normalize", "trim", "archive", "notify"}
+	if !reflect.DeepEqual(got, want) {
+		t.Fatalf("invalidateDownstreamSucceededStages() = %#v, want %#v", got, want)
+	}
+
+	for _, stageName := range want {
+		if m.Stages[stageName].Status != manifest.StatusStale {
+			t.Fatalf("%s status = %q, want stale", stageName, m.Stages[stageName].Status)
+		}
+	}
+	if m.Stages["analyze"].Status != manifest.StatusFailed {
+		t.Fatalf("analyze status = %q, want failed", m.Stages["analyze"].Status)
+	}
+	if m.Stages["prepare"].Status != manifest.StatusSucceeded {
+		t.Fatalf("prepare status = %q, want succeeded", m.Stages["prepare"].Status)
+	}
+}