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 {

internal/growth/report.go

@@ -64,6 +64,15 @@ func BuildDeterministicReport(input ReportInput) DeterministicReport {
 	wins := historicalWins(input.History, catalog, input.ForecastStart)
 	opportunities := buildOpportunities(wins, catalog, forecast, input.ForecastStart, 5)
 	risks := DetectRisks(forecast, 3)
+	if wins == nil {
+		wins = []EventEffect{}
+	}
+	if opportunities == nil {
+		opportunities = []Opportunity{}
+	}
+	if risks == nil {
+		risks = []Risk{}
+	}
 	bundle := EvidenceBundle{Opportunities: opportunities, Risks: risks}
 	report := DeterministicReport{Forecast: forecast, HistoricalWins: wins, Opportunities: opportunities, Risks: risks, FallbackBriefs: BuildFallback(bundle)}
 	if input.Peer != nil {