Add output validation against a defined JSON schema

internal/cli/merge_test.go

@@ -102,6 +102,9 @@ func TestMergeWritesMergedOutputAndReport(t *testing.T) {
 	if !equalStrings(gotModules, wantModules) {
 		t.Fatalf("report event order mismatch:\ngot  %v\nwant %v", gotModules, wantModules)
 	}
+	if !hasReportEvent(rpt, "postprocessing", "validate-output", "validated 3 output segment(s)") {
+		t.Fatal("expected validate-output report event")
+	}
 }
 
 func TestMergeTieBreakOrder(t *testing.T) {
@@ -153,6 +156,28 @@ func TestMergeTieBreakOrder(t *testing.T) {
 	}
 }
 
+func TestMergeValidateOutputBeforeAssignIDsFails(t *testing.T) {
+	dir := t.TempDir()
+	input := writeJSONFile(t, dir, "input.json", `{
+		"segments": [
+			{"start": 1, "end": 2, "text": "hello"}
+		]
+	}`)
+	output := filepath.Join(dir, "merged.json")
+
+	err := executeMerge(
+		"--input-file", input,
+		"--output-file", output,
+		"--postprocessing-modules", "validate-output,assign-ids",
+	)
+	if err == nil {
+		t.Fatal("expected validation error")
+	}
+	if !strings.Contains(err.Error(), "validate-output: segment 0 has id 0; want 1") {
+		t.Fatalf("unexpected error: %v", err)
+	}
+}
+
 func TestMergeDetectsOverlapGroups(t *testing.T) {
 	dir := t.TempDir()
 	inputA := writeJSONFile(t, dir, "a.json", `{