feat(Q5): NOAA sunset calculator (stdlib only)

tests/unit/test_solar.py

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+import datetime
+from vigilar.detection.solar import get_sunset
+
+
+def test_sunset_returns_time():
+    result = get_sunset(45.0, -85.0, datetime.date(2026, 6, 21))
+    assert isinstance(result, datetime.time)
+
+
+def test_sunset_equator():
+    result = get_sunset(0.0, 0.0, datetime.date(2026, 3, 20))
+    assert 17 <= result.hour <= 19
+
+
+def test_sunset_different_dates_vary():
+    d1 = get_sunset(45.0, -85.0, datetime.date(2026, 3, 1))
+    d2 = get_sunset(45.0, -85.0, datetime.date(2026, 9, 1))
+    assert d1 != d2

vigilar/detection/solar.py

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+"""Sunset calculation using NOAA solar equations. Stdlib only."""
+
+import datetime
+import math
+
+
+def get_sunset(latitude: float, longitude: float, date: datetime.date) -> datetime.time:
+    n = date.timetuple().tm_yday
+    gamma = 2 * math.pi / 365 * (n - 1)
+    eqtime = 229.18 * (
+        0.000075 + 0.001868 * math.cos(gamma) - 0.032077 * math.sin(gamma)
+        - 0.014615 * math.cos(2 * gamma) - 0.040849 * math.sin(2 * gamma)
+    )
+    decl = (
+        0.006918 - 0.399912 * math.cos(gamma) + 0.070257 * math.sin(gamma)
+        - 0.006758 * math.cos(2 * gamma) + 0.000907 * math.sin(2 * gamma)
+        - 0.002697 * math.cos(3 * gamma) + 0.00148 * math.sin(3 * gamma)
+    )
+    lat_rad = math.radians(latitude)
+    zenith = math.radians(90.833)
+    cos_ha = (
+        math.cos(zenith) / (math.cos(lat_rad) * math.cos(decl))
+        - math.tan(lat_rad) * math.tan(decl)
+    )
+    cos_ha = max(-1.0, min(1.0, cos_ha))
+    ha = math.degrees(math.acos(cos_ha))
+    sunset_minutes = 720 - 4 * (longitude - ha) - eqtime
+    sunset_minutes = sunset_minutes % 1440
+    hours = int(sunset_minutes // 60)
+    minutes = int(sunset_minutes % 60)
+    return datetime.time(hour=hours, minute=minutes)