Lighting: directional sun shadows from the day/night sun angle

DayNight.SunShadow returns a cell offset opposite the sun's east-west position, longest near sunrise/sunset (low sun) and zero at noon/night, tilted slightly south so shadows fall in front of occluders. LightmapBuilder gains a directional shadow pass (MaxShadowCells length cap, SunShadowStrength darkening); LightmapSystem feeds it the current sun shadow each rebuild. Tests cover SunShadow's day-arc behaviour and the builder's directional darkening.

src/MrGameEng.Graphics/Lighting/DayNight.cs

@@ -52,6 +52,26 @@ public sealed class DayNight
     /// <summary>Light intensity at a world point in <c>[0, 1]</c>. Global today; local (with shadows) later.</summary>
     public float SampleAt(Vector2 world) => Daylight;
 
+    /// <summary>
+    /// Offset, in grid cells, of the shadow an occluder casts under the current sun: opposite the
+    /// sun's east–west position and longest near sunrise/sunset (low sun), shrinking to zero at noon
+    /// and at night. Feeds the lightmap's directional shadow pass; <paramref name="maxLength"/> caps
+    /// the dawn/dusk shadow length. Tilted slightly "south" (down) so shadows fall in front of objects.
+    /// </summary>
+    public Vector2 SunShadow(float maxLength)
+    {
+        var day = (_calendar.DayProgress - 0.25f) / 0.5f; // daytime fraction over [06:00, 18:00]
+        if (day <= 0f || day >= 1f)
+        {
+            return Vector2.Zero; // night — no sun; the ambient floor handles darkness
+        }
+
+        var altitude = MathF.Sin(day * MathF.PI); // 0 at dawn/dusk, 1 at noon
+        var direction = new Vector2(2f * day - 1f, 0.4f); // sun east→west ⇒ shadow west→east, tilted south
+        direction.Normalize();
+        return direction * (maxLength * (1f - altitude));
+    }
+
     /// <summary>Ambient tint for the scene: night color at night, day color at noon, eased between.</summary>
     public Color Ambient
     {