fix #427: sky drawn without fog like retail; world fog range is the keyframe's authored MinWorldFog/MaxWorldFog

Two April stand-ins, neither registered, met at the horizon from altitude as
a hard line between the dome's rim and the fog-coloured clear:

1. sky.frag fogged every non-additive sky layer with a 0.2 floor. Retail's
   GameSky::Draw @0x00506FF0 disables fixed-function fog around the whole
   sky draw unless an AdminEnvirons fog override is active (SetFFFogEnable(
   LScape::m_override_enabled ? 1 : 0)); additive layers stay unfogged via
   SetFFFogAlphaDisabled(1) at D3DPolyRender::SetSurface 0x59c882. The sky
   pass now sets ApplyFog only for (override active && !additive), with no
   floor.
2. WorldRenderFrameBuilder overwrote the authored fog range with one derived
   from the streaming window (538..2189 m always). Retail sets FOGSTART/
   FOGEND straight from the keyframe's MinWorldFog/MaxWorldFog
   (SkyDesc::GetWorldFog @0x00500CE0 -> SetFFFogProperties @0x005A2F70) with
   no draw-distance scaling; zfar is a constant 4000 m. The builder now
   leaves SceneLightingUbo.Build's values alone; ACDREAM_FOG_START_MULT /
   _END_MULT are deleted from RuntimeOptions.

Guards: SkyFogRuleTests (source-level, the sky renderer has no hermetic
harness); sky.frag.spv re-pinned in VulkanShaderManifestTests with the
reason. Research note 2026-04-23-sky-fog.md carries a correction banner.
App hermetic 6,070/0, Core 4,707/0 (Release). Owner look gate owed: night
and rain fog are now retail's shorter authored ranges.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

tests/AcDream.App.Tests/Rendering/Sky/SkyFogRuleTests.cs

@@ -0,0 +1,86 @@
+using System;
+using System.IO;
+using Xunit;
+
+namespace AcDream.App.Tests.Rendering.Sky;
+
+/// <summary>
+/// Retail draws the sky with fixed-function fog DISABLED (GameSky::Draw
+/// @0x00506FF0: SetFFFogEnable(LScape::m_override_enabled ? 1 : 0) around
+/// the sky draw) and sets the world fog range straight from the keyframe's
+/// authored MinWorldFog/MaxWorldFog (SkyDesc::GetWorldFog @0x00500CE0 ->
+/// RenderDeviceD3D::SetFFFogProperties @0x005A2F70, no draw-distance
+/// scaling). Two April-2026 stand-ins contradicted that — a 0.2 "fog floor"
+/// on the dome and a fog range derived from the streaming window — and
+/// together showed as a hard line below the horizon from altitude
+/// (2026-08-23). These source guards keep both retail rules in place; the
+/// renderer itself needs a GPU and the installed DATs, so there is no
+/// hermetic behavioural harness for it.
+/// </summary>
+public sealed class SkyFogRuleTests
+{
+    [Fact]
+    public void SkyFragmentShaderFogsTheDomeOnlyThroughUApplyFogAndWithoutAFloor()
+    {
+        string frag = File.ReadAllText(Path.Combine(ShaderRoot(), "sky.frag"));
+        string code = StripLineComments(frag);
+
+        Assert.DoesNotContain("SKY_FOG_FLOOR", code, StringComparison.Ordinal);
+        Assert.DoesNotContain("max(vFogFactor", code, StringComparison.Ordinal);
+        Assert.Contains("if (uApplyFog > 0.5 && fogMode != 0)", code, StringComparison.Ordinal);
+        Assert.Contains("rgb = mix(uFogColor.rgb, rgb, vFogFactor);", code, StringComparison.Ordinal);
+    }
+
+    [Fact]
+    public void SkyRendererAppliesFogOnlyUnderAnAdminEnvironsOverrideAndNeverOnAdditiveLayers()
+    {
+        string source = File.ReadAllText(Path.Combine(
+            RepositoryRoot(), "src", "AcDream.App", "Rendering", "Sky", "SkyRenderer.cs"));
+        string code = StripLineComments(source);
+
+        Assert.Contains(
+            "_params.ApplyFog = environOverrideActive && !sub.DisableFog ? 1f : 0f;",
+            code,
+            StringComparison.Ordinal);
+        Assert.DoesNotContain("_params.ApplyFog = sub.DisableFog ? 0f : 1f;", code, StringComparison.Ordinal);
+    }
+
+    [Fact]
+    public void WorldFrameBuilderLeavesTheAuthoredFogRangeAlone()
+    {
+        string source = File.ReadAllText(Path.Combine(
+            RepositoryRoot(), "src", "AcDream.App", "Rendering", "WorldRenderFrameBuilder.cs"));
+        string code = StripLineComments(source);
+
+        // SceneLightingUbo.Build writes FogParams.xy from AtmosphereSnapshot
+        // (the keyframe's authored range, SceneLightingUboTests pins that);
+        // nothing downstream may overwrite them from the streaming window.
+        Assert.DoesNotContain("ubo.FogParams = new Vector4(", code, StringComparison.Ordinal);
+        Assert.DoesNotContain("FogEndMultiplier", code, StringComparison.Ordinal);
+        Assert.DoesNotContain("FogStartMultiplier", code, StringComparison.Ordinal);
+    }
+
+    private static string StripLineComments(string source)
+    {
+        var lines = source.Split('\n');
+        for (int i = 0; i < lines.Length; i++)
+        {
+            int idx = lines[i].IndexOf("//", StringComparison.Ordinal);
+            if (idx >= 0)
+                lines[i] = lines[i][..idx];
+        }
+        return string.Join('\n', lines);
+    }
+
+    private static string ShaderRoot() =>
+        Path.Combine(RepositoryRoot(), "src", "AcDream.App", "Rendering", "Shaders");
+
+    private static string RepositoryRoot()
+    {
+        var directory = new DirectoryInfo(AppContext.BaseDirectory);
+        while (directory is not null && !File.Exists(Path.Combine(directory.FullName, "AcDream.slnx")))
+            directory = directory.Parent;
+        return directory?.FullName
+            ?? throw new InvalidOperationException("Could not locate repository root.");
+    }
+}