feat(world): Phase G.1 data model — dat-accurate SkyKeyframe + WeatherSystem

Expand the SkyKeyframe record with retail-exact fog fields (FogStart,
FogEnd, FogMode) per r12 §5. The existing FogDensity field is retained
for backwards compat with tests that pin it; new shipping code reads
FogStart / FogEnd / FogMode directly.

Add WeatherSystem (WeatherKind + EnvironOverride enum + 10s transition
ease + deterministic per-day-index roll) matching r12 §6.1. Roll weights
are ~60% Clear / 20% Overcast / 12% Rain / 5% Snow / 3% Storm — tuned
against retail observations. Storm mode triggers lightning flashes
every 8–30 s via an exponential-decay (200ms τ) flash level that the
shader consumes as an additive scene bump.

Add SkyDescLoader that parses the Region dat (0x13000000) into
LoadedSkyDesc — DayGroupData with SkyObjectData (visibility window +
arc sweep), per-keyframe SkyObjectReplaceData, and a shader-ready
SkyStateProvider builder. Sun/ambient colors are pre-multiplied by
DirBright/AmbBright so the shader never needs to know about retail's
scalar brightness field.

19 new tests (weather determinism, transition ease, environ override
tint, flash decay, dat-load conversion with fog + pre-mult colors).

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

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

@@ -0,0 +1,136 @@
+using System.Collections.Generic;
+using System.Numerics;
+using AcDream.Core.World;
+using DatReaderWriter.DBObjs;
+using DatReaderWriter.Enums;
+using DatReaderWriter.Types;
+using Xunit;
+
+namespace AcDream.Core.Tests.World;
+
+public sealed class SkyDescLoaderTests
+{
+    /// <summary>
+    /// Hand-build a Region with a minimal sky descriptor to feed the
+    /// loader without needing real dat bytes. The LoadFromRegion
+    /// separator exists precisely for this — keeps the parsing logic
+    /// testable independent of DatCollection.
+    /// </summary>
+    private static Region MakeRegion(float dirBright, byte rBgrOrder)
+    {
+        var region = new Region();
+        region.PartsMask = PartsMask.HasSkyInfo;
+
+        var sky = new SkyDesc
+        {
+            TickSize = 1.0,
+            LightTickSize = 2.0,
+        };
+
+        var dg = new DayGroup
+        {
+            ChanceOfOccur = 1.0f,
+        };
+
+        var time = new SkyTimeOfDay
+        {
+            Begin         = 0.5f,
+            DirBright     = dirBright,
+            DirHeading    = 180f,
+            DirPitch      = 70f,
+            DirColor      = new ColorARGB { Blue = 0, Green = 0, Red = rBgrOrder, Alpha = 255 },
+            AmbBright     = 0.4f,
+            AmbColor      = new ColorARGB { Blue = 100, Green = 100, Red = 100, Alpha = 255 },
+            MinWorldFog   = 120f,
+            MaxWorldFog   = 400f,
+            WorldFogColor = new ColorARGB { Blue = 50, Green = 50, Red = 50, Alpha = 255 },
+            WorldFog      = 1,  // Linear
+        };
+
+        dg.SkyTime.Add(time);
+        sky.DayGroups.Add(dg);
+        region.SkyInfo = sky;
+
+        return region;
+    }
+
+    [Fact]
+    public void LoadFromRegion_ConvertsFogFields()
+    {
+        var region = MakeRegion(dirBright: 1.5f, rBgrOrder: 200);
+        var loaded = SkyDescLoader.LoadFromRegion(region);
+
+        Assert.NotNull(loaded);
+        Assert.Equal(1.0, loaded!.TickSize);
+        Assert.Single(loaded.DayGroups);
+        var grp = loaded.DayGroups[0];
+        Assert.Single(grp.SkyTimes);
+
+        var kf = grp.SkyTimes[0].Keyframe;
+        Assert.Equal(120f, kf.FogStart);
+        Assert.Equal(400f, kf.FogEnd);
+        Assert.Equal(FogMode.Linear, kf.FogMode);
+    }
+
+    [Fact]
+    public void LoadFromRegion_SunColor_IsPrepreMultipliedByBrightness()
+    {
+        var region = MakeRegion(dirBright: 1.5f, rBgrOrder: 200);
+        var loaded = SkyDescLoader.LoadFromRegion(region);
+        Assert.NotNull(loaded);
+
+        var kf = loaded!.DayGroups[0].SkyTimes[0].Keyframe;
+        // R was 200/255 ≈ 0.784, times dirBright 1.5 = 1.176
+        Assert.InRange(kf.SunColor.X, 1.17f, 1.19f);
+    }
+
+    [Fact]
+    public void LoadFromRegion_NoSkyInfo_ReturnsNull()
+    {
+        var region = new Region { PartsMask = 0 };
+        Assert.Null(SkyDescLoader.LoadFromRegion(region));
+    }
+
+    [Fact]
+    public void BuildDefaultProvider_FromDatKeyframes_SupportsInterpolation()
+    {
+        var region = MakeRegion(dirBright: 1.0f, rBgrOrder: 255);
+        var loaded = SkyDescLoader.LoadFromRegion(region)!;
+        var provider = loaded.BuildDefaultProvider();
+
+        // Exactly one keyframe: interpolation at any t returns it.
+        var s = provider.Interpolate(0.1f);
+        Assert.InRange(s.SunColor.X, 0.99f, 1.01f);
+    }
+
+    [Fact]
+    public void SkyObjectData_IsVisible_HandlesWrap()
+    {
+        var obj = new SkyObjectData
+        {
+            BeginTime = 0.9f,  // wraps across midnight
+            EndTime   = 0.1f,
+        };
+
+        Assert.True(obj.IsVisible(0.95f));   // near end of day
+        Assert.True(obj.IsVisible(0.05f));   // just after midnight
+        Assert.False(obj.IsVisible(0.5f));   // mid-day (not visible)
+    }
+
+    [Fact]
+    public void SkyObjectData_CurrentAngle_LerpsAcrossWindow()
+    {
+        var obj = new SkyObjectData
+        {
+            BeginTime  = 0.25f,
+            EndTime    = 0.75f,
+            BeginAngle = 0f,
+            EndAngle   = 180f,
+        };
+
+        // Middle of the window → 90°.
+        Assert.Equal(90f, obj.CurrentAngle(0.5f), precision: 2);
+        // At begin → begin angle.
+        Assert.Equal(0f,  obj.CurrentAngle(0.25f), precision: 2);
+    }
+}