fix(sky): retail-faithful sun-vector magnitude for SunColor / AmbientColor

Two independent investigations (in-house decomp re-check + two
external agent reports) converged on the same root cause for the
"too blue-white sky" symptom:

acdream computed SunColor = DirColor × DirBright and AmbientColor =
AmbColor × AmbBright. Retail computes them from the magnitude of a
specially-shaped sun vector instead. Per the named retail decomp:

  SkyDesc::GetLighting at 0x00500ac9 (decomp 261343-261353):
    sunVec.x = sin(H_rad) × DirBright × cos(P_rad)
    sunVec.y = cos(P_rad)                    ← NOT scaled by DirBright
    sunVec.z = DirBright × sin(P_rad)

  PrimD3DRender::UpdateLightsInternal at 0x0059b57c (decomp 424118):
    D3DLIGHT9.Diffuse.r = sunlight_color.r × sqrt(x²+y²+z²)

  SmartBox::SetWorldAmbientLight callsite at 0x0050560b (decomp 267117):
    SetWorldAmbientLight(sqrt(|sunVec|²) × 0.2 + ambient_level, ...)

Y stays unscaled by DirBright on purpose, so |sunVec| ≠ DirBright in
general — the magnitude varies with sun pitch/heading. That's what
gives retail's "sun feels stronger when it's overhead, ambient warms
up at midday" behavior we were missing.

Added SkyStateProvider.RetailSunVector(kf) that builds the vector
verbatim. SkyKeyframe.SunColor / AmbientColor now compose via |sunVec|.
SunDirectionFromKeyframe normalizes the same vector (replaces our
geometrically-clean spherical convention which didn't match retail's
deliberate Y-decoupled-from-heading shape).

Tests:
- Replaced the linear-interp assumption in
  Interpolate_BetweenKeyframes_LerpsColors with a test on the RAW
  inputs (DirColor, AmbBright, etc.) — those still lerp linearly;
  the composite SunColor doesn't, intentionally.
- Added 4 golden-value tests for the new formulas
  (RetailSunVector_AtZenith, _AtHorizonNorth,
  SunColor_UsesRetailMagnitudeNotDirBrightDirectly,
  AmbientColor_BoostsByTwentyPercentOfSunVectorLength).
- Updated stale LoadFromRegion_SunColor_IsPrepreMultipliedByBrightness
  test to LoadFromRegion_SunColor_UsesRetailSunVectorMagnitude
  with the new expected magnitude.

User visually verified — acdream's sky shifted from blue-white toward
the warm tint retail shows at the same keyframe.

1227 tests pass.

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

@@ -73,15 +73,26 @@ public sealed class SkyDescLoaderTests
     }
 
     [Fact]
-    public void LoadFromRegion_SunColor_IsPrepreMultipliedByBrightness()
+    public void LoadFromRegion_SunColor_UsesRetailSunVectorMagnitude()
     {
+        // The loader stores DirColor and DirBright RAW. The SunColor property
+        // composes them via |sunVec| per retail's UpdateLightsInternal at
+        // 0x59b57c (decomp 424118) — the diffuse magnitude is sqrt(x²+y²+z²)
+        // where the sun vector is built from heading/pitch/brightness with
+        // Y unscaled by brightness (decomp 261352).
+        //
+        // For this region:  H=180°, P=70°, B=1.5
+        //   sunVec = (sin(180)*1.5*cos(70), cos(70), 1.5*sin(70))
+        //          = (0, 0.342, 1.410)
+        //   |sunVec| = sqrt(0 + 0.117 + 1.988) = 1.4509
+        // DirColor.X = 200/255 = 0.7843
+        // SunColor.X = 0.7843 × 1.4509 = 1.138
         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);
+        Assert.InRange(kf.SunColor.X, 1.13f, 1.15f);
     }
 
     [Fact]