feat(world): Phase G.1 — debug-time override tests + clear-color clamp

Small polish commit:
- Clamp ClearColor inputs to [0, 1] because retail keyframes store
  sun/fog colors pre-multiplied by their brightness scalars, which can
  exceed 1.0; some drivers treat ClearColor > 1 as a saturate-bright
  hint and produce visible color shifts at the edges.
- 4 new tests cover WorldTimeService.SetDebugTime / ClearDebugTime /
  SyncFromServer-clears-override / SetProvider hot-swap.

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

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

@@ -0,0 +1,58 @@
+using System;
+using AcDream.Core.World;
+using Xunit;
+
+namespace AcDream.Core.Tests.World;
+
+public sealed class WorldTimeDebugTests
+{
+    [Fact]
+    public void SetDebugTime_OverridesDayFraction()
+    {
+        var service = new WorldTimeService(SkyStateProvider.Default());
+        service.SyncFromServer(0);  // midnight
+
+        service.SetDebugTime(0.5f);   // force noon
+        Assert.InRange(service.DayFraction, 0.499, 0.501);
+    }
+
+    [Fact]
+    public void ClearDebugTime_RestoresServerTime()
+    {
+        var service = new WorldTimeService(SkyStateProvider.Default());
+        service.SyncFromServer(DerethDateTime.DayTicks * 0.25); // dawn
+        service.SetDebugTime(0.5f);
+        service.ClearDebugTime();
+
+        Assert.InRange(service.DayFraction, 0.24, 0.26);
+    }
+
+    [Fact]
+    public void SyncFromServer_ClearsDebugOverride()
+    {
+        var service = new WorldTimeService(SkyStateProvider.Default());
+        service.SetDebugTime(0.75f);
+        service.SyncFromServer(0);  // midnight — this should clear the override
+
+        Assert.InRange(service.DayFraction, 0.0, 0.01);
+    }
+
+    [Fact]
+    public void SetProvider_AcceptsNewKeyframes()
+    {
+        var service = new WorldTimeService(SkyStateProvider.Default());
+        var custom = new SkyStateProvider(new[]
+        {
+            new SkyKeyframe(
+                Begin: 0f,
+                SunHeadingDeg: 0f,
+                SunPitchDeg: 90f,
+                SunColor: System.Numerics.Vector3.One,
+                AmbientColor: System.Numerics.Vector3.One,
+                FogColor: System.Numerics.Vector3.Zero,
+                FogDensity: 0f),
+        });
+        service.SetProvider(custom);
+        Assert.Equal(1, custom.KeyframeCount);
+    }
+}