Add forecast daypart derivation

internal/forecast/testdata/daily_bundle.json

@@ -0,0 +1,66 @@
+{
+  "fetchedAt": "2026-05-29T15:00:00Z",
+  "hourly": {
+    "issuedAt": "2026-05-29T10:30:00-05:00",
+    "product": "hourly",
+    "periods": [
+      {
+        "startTime": "2026-05-29T06:00:00-05:00",
+        "endTime": "2026-05-29T07:00:00-05:00",
+        "textDescription": "Showers and thunderstorms",
+        "temperatureF": 66,
+        "apparentTemperatureF": 67,
+        "probabilityOfPrecipitationPercent": 80,
+        "windSpeedMph": 18,
+        "windGustMph": 32
+      },
+      {
+        "startTime": "2026-05-29T14:00:00-05:00",
+        "endTime": "2026-05-29T15:00:00-05:00",
+        "textDescription": "Mostly sunny",
+        "temperatureF": 88,
+        "apparentTemperatureF": 91,
+        "probabilityOfPrecipitationPercent": 10,
+        "windSpeedMph": 10,
+        "windGustMph": 16
+      }
+    ]
+  },
+  "narrative": {
+    "issuedAt": "2026-05-29T10:30:00-05:00",
+    "product": "narrative",
+    "periods": [
+      {
+        "startTime": "2026-05-29T06:00:00-05:00",
+        "endTime": "2026-05-29T18:00:00-05:00",
+        "name": "Today",
+        "textDescription": "Morning storms, then partly sunny."
+      }
+    ]
+  },
+  "alerts": {
+    "alerts": [
+      {
+        "event": "Flood Watch",
+        "headline": "Flooding possible",
+        "severity": "Moderate",
+        "effective": "2026-05-29T05:00:00-05:00",
+        "expires": "2026-05-29T09:00:00-05:00"
+      }
+    ]
+  },
+  "discussion": {
+    "product": "discussion",
+    "issuedAt": "2026-05-29T09:25:00-05:00",
+    "keyMessages": [
+      "Storms are most likely during the morning."
+    ]
+  },
+  "sources": [
+    {
+      "name": "hourly",
+      "endpoint": "/forecast/hourly",
+      "fetchedAt": "2026-05-29T15:00:00Z"
+    }
+  ]
+}