Implement derived forecast modules

internal/facts/facts.go

@@ -75,6 +75,7 @@ type DerivedFacts struct {
 	AlertOverlaps               []forecast.AlertOverlap
 	DailySummaries              []forecast.DailySummary
 	DaypartSummaries            []forecast.DaypartSummary
+	PrecipTiming                forecast.PrecipTiming
 	StormWindowSummary          *forecast.DaypartSummary
 }
 
@@ -101,6 +102,7 @@ func BuildDerived(req BuildDerivedRequest) (DerivedFacts, error) {
 		ValidPeriodDailyPeriods:     forecast.SelectHourlyPeriods(req.Collected.Daily, period),
 		AlertOverlaps:               forecast.AlertOverlaps(req.Collected.Alerts, period),
 	}
+	derived.PrecipTiming = forecast.BuildPrecipTiming(derived.ValidPeriodHourlyPeriods)
 
 	switch req.Resolved.Definition.ID {
 	case report.DailyToday, report.DailyTomorrow:

internal/facts/facts_test.go

@@ -66,6 +66,12 @@ func TestBuildDerivedDailySlicesDaypartsAndAlerts(t *testing.T) {
 	if len(derived.DaypartSummaries) != 3 {
 		t.Fatalf("DaypartSummaries length = %d, want 3", len(derived.DaypartSummaries))
 	}
+	if derived.PrecipTiming.FirstPrecipitation == nil || derived.PrecipTiming.FirstPrecipitation.Time.Format(time.RFC3339) != "2026-05-29T08:00:00-05:00" {
+		t.Fatalf("PrecipTiming.FirstPrecipitation = %#v, want valid-period rain start", derived.PrecipTiming.FirstPrecipitation)
+	}
+	if !derived.PrecipTiming.ThunderMentioned {
+		t.Fatal("PrecipTiming.ThunderMentioned = false, want true")
+	}
 	morning := derived.DailySummaries[0].Dayparts[0]
 	if len(morning.HourlyPeriods) != 1 || morning.MaxPrecipitationProbability == nil || morning.MaxPrecipitationProbability.Value != 60 {
 		t.Fatalf("morning summary = %#v, want sliced hour with precip max", morning)