fix(sky): retail-faithful keyframe lerp — separate-channel color/bright

Retail's SkyDesc::GetLighting at 0x00500ac9 (decomp lines 261317-261331)
lerps each color channel and the brightness scalar SEPARATELY, then
multiplies post-lerp:

  arg4.r = lerp(k1.amb_color.r, k2.amb_color.r, u)
  arg4.g = lerp(k1.amb_color.g, k2.amb_color.g, u)
  arg4.b = lerp(k1.amb_color.b, k2.amb_color.b, u)
  arg3   = lerp(k1.amb_bright, k2.amb_bright, u)
  final  = (arg4.rgb * arg3, ...)

acdream pre-multiplied (color × bright) at LOAD time
(`SkyDescLoader.cs:558-559`) and then lerped the product. For any
keyframe pair where both color and brightness change, the two are
mathematically distinct. Example, k1=(white, b=0.5) k2=(black, b=1.0)
at u=0.5:
  - retail: color=gray(0.5), bright=0.75 → final = (0.375, 0.375, 0.375)
  - acdream: lerp((0.5,0.5,0.5), (0,0,0), 0.5) = (0.25, 0.25, 0.25)

For Rainy/Cloudy DayGroups transitioning between dim and bright
keyframes, this contributes to subtle brightness divergence vs retail.

Refactor:
  SkyKeyframe stores DirColor / DirBright / AmbColor / AmbBright
    SEPARATELY (raw, not pre-multiplied).
  Computed properties SunColor and AmbientColor return the
    post-multiplied product, keeping the shader uniform interface
    (uSunColor / uAmbientColor) unchanged.
  SkyStateProvider.Interpolate lerps each raw channel, then constructs
    a new SkyKeyframe whose computed properties yield the correct
    post-lerp multiply.
  SkyDescLoader now stores raw values without pre-multiplying.
  GameWindow comment updated; no functional change there.
  Default factory + tests updated to use the new constructor parameters
    with DirBright=AmbBright=1.0 (preserving exact existing behavior).

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

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

@@ -56,8 +56,10 @@ public sealed class SkyStateTests
             Begin: 0.5f,
             SunHeadingDeg: 180f,  // south
             SunPitchDeg:   70f,
-            SunColor:      Vector3.One,
-            AmbientColor:  Vector3.One,
+            DirColor:      Vector3.One,
+            DirBright:     1f,
+            AmbColor:      Vector3.One,
+            AmbBright:     1f,
             FogColor:      Vector3.One,
             FogDensity:    0.001f);
 

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

@@ -58,8 +58,10 @@ public sealed class WorldTimeDebugTests
                 Begin: 0f,
                 SunHeadingDeg: 0f,
                 SunPitchDeg: 90f,
-                SunColor: System.Numerics.Vector3.One,
-                AmbientColor: System.Numerics.Vector3.One,
+                DirColor: System.Numerics.Vector3.One,
+                DirBright: 1f,
+                AmbColor: System.Numerics.Vector3.One,
+                AmbBright: 1f,
                 FogColor: System.Numerics.Vector3.Zero,
                 FogDensity: 0f),
         });