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>

src/AcDream.App/Rendering/Shaders/sky.frag

@@ -57,19 +57,24 @@ void main() {
 
     vec3 rgb = sampled.rgb * vTint;
 
-    // SHOULD-FIX S2/mech NOTE N-3 (OP4 review-fix round, 2026-08-11):
-    // PlayerOption DisableDistanceFog forces FogMode.Off (uFogParams.w
-    // == 0) — mesh_modern.frag/terrain_modern.frag both gate their own
-    // fog blend on this same word (`if (mode == 0) return lit;`), but the
-    // sky dome's blend here read only uApplyFog (the CPU per-submesh
-    // "is this layer foggable at all" flag) and never uFogParams.w, so
-    // toggling the option stopped terrain/objects fading into fog while
-    // the dome's horizon band kept blending toward fog color.
+    // Retail draws the sky with fixed-function fog DISABLED:
+    // GameSky::Draw @0x00506FF0 saves GetFFFogEnable, calls
+    // SetFFFogEnable(LScape::m_override_enabled ? 1 : 0), draws every sky
+    // object, and restores it. So the dome is fogged ONLY while an
+    // AdminEnvirons fog override is active; the horizon tint otherwise
+    // comes from the authored dome textures and the keyframe tint, and the
+    // frame's fog-coloured clear meets the dome's own colour at the rim.
+    // The CPU sets uApplyFog = 1 only for (override active && surface not
+    // Additive — SetFFFogAlphaDisabled(1) at D3DPolyRender::SetSurface
+    // 0x59c882). uFogParams.w == 0 is PlayerOption DisableDistanceFog
+    // (LScape::m_fFogEnabled -> SetFFFogUserDisabled), honoured here like
+    // every other fogged pass. The 2026-04-27 "SKY_FOG_FLOOR = 0.2" clamp
+    // was a stand-in for this rule before it was read from retail; it
+    // left the dome's rim at 80 % fog against a 100 % fog clear, which
+    // showed as a hard line below the horizon from altitude (2026-08-23).
     int fogMode = int(uFogParams.w);
     if (uApplyFog > 0.5 && fogMode != 0) {
-        const float SKY_FOG_FLOOR = 0.2;
-        float skyFogFactor = max(vFogFactor, SKY_FOG_FLOOR);
-        rgb = mix(uFogColor.rgb, rgb, skyFogFactor);
+        rgb = mix(uFogColor.rgb, rgb, vFogFactor);
     }
 
     float flash = uFogParams.z;

src/AcDream.App/Rendering/Shaders/spv/shaders.manifest.json

@@ -316,7 +316,7 @@
         },
         {
           "stage": "frag",
-          "sourceSha256": "2ddf210d69b0c4a3c0870eecfb0ccba2097d93b365729739402bfe91e3b120d0",
+          "sourceSha256": "b1f3b924af2040c909337c02461ed6795b5320fb04b5bb371ce1546feead567c",
           "compiled": true
         }
       ]

src/AcDream.App/Rendering/Sky/SkyRenderer.cs

@@ -372,18 +372,19 @@ public sealed partial class SkyRenderer : IDisposable
                 // with texture alpha and keyframe transparency.
                 _params.SurfOpacity = sub.SurfOpacity;
 
-                // Retail D3DPolyRender::SetSurface at 0x59c882 calls
-                // SetFFFogAlphaDisabled(1) when the Additive flag (0x10000)
-                // is set on the Surface — so the sun, moon, stars, and any
-                // additive cloud sheet are drawn WITHOUT fog. Skipping fog
-                // on additive surfaces keeps the sun bright at horizon
-                // dusk/dawn (where fog would otherwise dim it to fog color).
-                // Non-additive sky meshes (the dome/background layers)
-                // still mix toward keyframe fog with the floor mitigation
-                // in sky.frag. That restores the broad green/purple Rainy
-                // DayGroup tint behind the cloud sheet while raw-additive
-                // 0x08000023 remains unfogged and keeps the pink detail.
-                _params.ApplyFog = sub.DisableFog ? 0f : 1f;
+                // Retail GameSky::Draw @0x00506FF0 draws the whole sky with
+                // fixed-function fog DISABLED unless an AdminEnvirons fog
+                // override is active (SetFFFogEnable(LScape::
+                // m_override_enabled ? 1 : 0) around the sky draw). Within
+                // that, D3DPolyRender::SetSurface at 0x59c882 calls
+                // SetFFFogAlphaDisabled(1) for Additive surfaces (sun,
+                // moon, stars, additive cloud sheets), so those are never
+                // fogged. The dome's horizon tint is the authored texture
+                // plus keyframe tint — not fog — which is why the frame's
+                // fog-coloured clear meets the dome cleanly at its rim.
+                // (Until 2026-08-23 every non-additive layer was fogged
+                // with a 0.2 floor; see sky.frag for the symptom.)
+                _params.ApplyFog = environOverrideActive && !sub.DisableFog ? 1f : 0f;
 
                 // Sky meshes need per-object wrap mode driven by the
                 // mesh's authored UV range, not by TexVelocity:

src/AcDream.App/Rendering/WorldRenderFrameBuilder.cs

@@ -500,17 +500,18 @@ internal sealed class RuntimeWorldFrameEnvironmentPreparation
             in atmosphere,
             camera.Position,
             (float)_worldTime.DayFraction);
-        float fogStart = _ranges.NearRadius
-            * LandblockSize
-            * _options.FogStartMultiplier;
-        float fogEnd = _ranges.FarRadius
-            * LandblockSize
-            * _options.FogEndMultiplier;
-        ubo.FogParams = new Vector4(
-            fogStart,
-            fogEnd,
-            ubo.FogParams.Z,
-            ubo.FogParams.W);
+        // FogParams.xy stay the keyframe's authored MinWorldFog/MaxWorldFog
+        // that SceneLightingUbo.Build wrote (AtmosphereSnapshot.FogStart/
+        // FogEnd). Retail sets D3DRS_FOGSTART/FOGEND straight from those
+        // (SkyDesc::GetWorldFog @0x00500CE0 -> LScape::UseTime ->
+        // RenderDeviceD3D::SetFFFogProperties @0x005A2F70) with no draw-
+        // distance scaling; its far plane is a constant 4000 m
+        // (Render::zfar @0x0081EC88). Until 2026-08-23 this site overwrote
+        // them with a range derived from the streaming window (0.7 x near
+        // radius .. 0.95 x far radius = 538..2189 m at every hour and
+        // weather), which thinned retail's night (0..400 m) and rain
+        // (150..1500 m) fog and hid the fact that the sky pass was being
+        // fogged at all (see SkyRenderer / sky.frag).
         _lightingUbo?.Upload(ubo);
 
         RenderingDiagnostics.EmitLight(