fix: unblock first growth report generation

internal/growth/analytics_test.go

@@ -1,6 +1,8 @@
 package growth
 
 import (
+	"bytes"
+	"encoding/json"
 	"testing"
 	"time"
 )
@@ -194,6 +196,19 @@ func TestBuildDeterministicReportLimitsAndOrdersOpportunities(t *testing.T) {
 	}
 }
 
+func TestBuildDeterministicReportSerializesEmptyCollectionsAsArrays(t *testing.T) {
+	report := BuildDeterministicReport(ReportInput{ForecastStart: time.Date(2026, 7, 1, 0, 0, 0, 0, time.UTC), HorizonDays: 90})
+	encoded, err := json.Marshal(report)
+	if err != nil {
+		t.Fatal(err)
+	}
+	for _, field := range []string{`"historicalWins":[]`, `"opportunities":[]`, `"risks":[]`, `"fallbackBriefs":[]`} {
+		if !bytes.Contains(encoded, []byte(field)) {
+			t.Fatalf("report must encode %s as an array: %s", field, encoded)
+		}
+	}
+}
+
 func TestFingerprintTreatsEquivalentForecastDatesEqually(t *testing.T) {
 	tehran, err := time.LoadLocation("Asia/Tehran")
 	if err != nil {