refactor(runtime): own world environment state

tests/AcDream.Core.Tests/World/SkyDescLoaderTests.cs

@@ -10,6 +10,39 @@ namespace AcDream.Core.Tests.World;
 
 public sealed class SkyDescLoaderTests
 {
+    [Theory]
+    [InlineData(20, 10, 0, 16)]
+    [InlineData(20, 10, 83, 5)]
+    [InlineData(20, 116, 83, 18)]
+    [InlineData(20, 127, 359, 4)]
+    public void DayGroupSelector_MatchesNamedRetailGoldenValues(
+        int count,
+        int absoluteYear,
+        int dayOfYear,
+        int expected)
+    {
+        Assert.Equal(
+            expected,
+            SkyDayGroupSelector.SelectIndex(
+                count,
+                absoluteYear,
+                daysPerYear: 360,
+                dayOfYear));
+    }
+
+    [Fact]
+    public void DayGroupSelector_ForcedIndexIsExplicitInput()
+    {
+        Assert.Equal(
+            7,
+            SkyDayGroupSelector.SelectIndex(
+                20,
+                absoluteYear: 10,
+                daysPerYear: 360,
+                dayOfYear: 0,
+                forcedIndex: 7));
+    }
+
     /// <summary>
     /// Hand-build a Region with a minimal sky descriptor to feed the
     /// loader without needing real dat bytes. The LoadFromRegion