Clean up and normalize prompt output modules

internal/changes/daily.go

@@ -84,7 +84,8 @@ type dailySummaryStanza struct {
 
 type daypartSummaryStanza struct {
 	Date           string `json:"date,omitempty"`
-	Period         string `json:"period,omitempty"`
+	PeriodBegins   string `json:"period_begins,omitempty"`
+	PeriodEnds     string `json:"period_ends,omitempty"`
 	TempRangeF     string `json:"temp_range_f,omitempty"`
 	MaxPopPercent  *int   `json:"max_pop_percent,omitempty"`
 	MaxPopTime     string `json:"max_pop_time,omitempty"`

internal/changes/daily_test.go

@@ -96,7 +96,13 @@ func dailySnapshot(t *testing.T, low int, high int, precip int, precipTime strin
 			DailyPrecipitationProbability: &precip,
 		}},
 		module.Output{ID: module.DerivedDaypartSummaries, StanzaName: "derived_daypart_summaries", Value: map[string]daypartSummaryStanza{
-			"morning": {Date: "2026-05-29", Period: "2026-05-29 at 6:00 AM to 2026-05-29 at 10:00 AM", TempRangeF: "60-70", Snow: snow},
+			"morning": {
+				Date:         "2026-05-29",
+				PeriodBegins: "2026-05-29 at 6:00 AM",
+				PeriodEnds:   "2026-05-29 at 10:00 AM",
+				TempRangeF:   "60-70",
+				Snow:         snow,
+			},
 		}},
 		module.Output{ID: module.PrecipTiming, StanzaName: "precip_timing", Value: precipTimingStanza{MaxPopPercent: &precip, MaxPopTime: precipTime}},
 		module.Output{ID: module.AlertDigest, StanzaName: "alert_digest", Value: alertDigestStanza{Relevant: relevant}},

internal/changes/three_day_test.go

@@ -32,7 +32,8 @@ func outlookSnapshot(t *testing.T, date string, tempRange string, precip int, pr
 	return snapshot(t, module.Output{ID: module.DerivedDaypartSummaries, StanzaName: "derived_daypart_summaries", Value: map[string]daypartSummaryStanza{
 		date + "_morning": {
 			Date:          date,
-			Period:        date + " at 6:00 AM to " + date + " at 10:00 AM",
+			PeriodBegins:  date + " at 6:00 AM",
+			PeriodEnds:    date + " at 10:00 AM",
 			TempRangeF:    tempRange,
 			MaxPopPercent: &precip,
 			MaxPopTime:    precipTime,