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
         }
       ]