fix(render): detail overlay is fogged after the combine like retail; VM1 review fixes

Opus dual-lens review of 05970306 + 388457a7 (APPROVE WITH FIXES). Four
items, all landed:

1. FOG (behavioural). Retail's D3D fixed-function fog stage runs AFTER the
   texture-stage pipeline, so the detail contribution must be fogged, not
   just the base. mesh_modern.frag already fogs the base colour
   (applyFog(rgb, vWorldPos)) before mesh_detail's replay draws over it;
   mesh_detail.frag previously emitted raw detail.rgb, understating fog by
   f*a*(fog-detail). Fix: mesh_detail.vert now outputs vWorldPos (mirroring
   mesh_modern.vert); mesh_detail.frag declares the identical SceneLighting
   UBO and applyFog function (copied verbatim, same binding/std140/math) and
   fogs detail.rgb before emitting it. This collapses algebraically to
   retail's fog-after-combine order:
     (1-a)*mix(base,fog,f) + a*mix(detail,fog,f) = mix(lerp(base,detail,a),fog,f)
   RetailDetailTextureContract gains ExpectedFogged(base,detail,opacity,fog,
   fogFactor); RetailDetailTextureContractTests pins the identity across 200
   random samples within 1e-6.

2. EnvCellRenderer.Rhi.cs's DrawEnvCell-category comment still said "apply
   the 10-50 m positive-view-depth fade" — a stale claim from before VM1
   removed the fade. Replaced with the mip-chain attenuation statement that
   mesh_detail.vert's header comment already carries.

3. Added the test the VM1 contract required but never had: TerrainAtlas
   .TryCreateDetailTexture uploads a full mip chain (MipLevelCount ==
   RhiWorldTextureArray.MipLevelsFor(w,h), GenerateMipChain called) and
   registers with the repeat/linear world sampler, not single-level or
   clamped. Drives the private method directly (reflection) against a
   synthetic PFID_A8R8G8B8 RenderSurface through a minimal in-memory
   IDatReaderWriter fake, so the lane stays hermetic (no installed DAT).

4. #226 pseudocode note: noted that retail's stage-1 OUTPUT alpha
   (MODULATE(TEXTURE, CURRENT), 0x0059c549) — the framebuffer blend weight a
   delayed-alpha subset composites with — is not modelled; acdream instead
   draws a second pass weighted by detail.a*diffuseAlpha. Identical for
   opaque subsets, a bounded difference on translucent building/EnvCell
   subsets already covered by the existing AP-34 shared-alpha-queue
   divergence row. Also qualified the tmpmaterial.Diffuse.a = 1f (0x0059cb99)
   citation to name its exact branch (burnedInStaticLights < 0 &&
   *(render_device+0x7e4) == 0); the other branch leaves diffuse FromVertex,
   but the opaque->1 / fading->opacity mapping still holds either way.

Nit also folded in: EnvCellRendererTests' new SubmitRhi instance-alpha test
is now a [Theory] over WbRenderPass.Opaque and .Transparent, pinning the
bind-before-first-draw invariant on both passes.

Regenerated mesh_detail's committed SPIR-V and the shader manifest
(tools/compile-shaders.ps1); no other shader pair changed.

Verified: dotnet build AcDream.slnx -c Release (0 warnings, 0 errors);
dotnet test on AcDream.App.Tests (Release, hermetic lanes) green, including
the shader manifest tests explicitly; AcDream.Core.Tests unaffected/green.

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

src/AcDream.App/Rendering/RetailDetailTextureContract.cs

@@ -77,4 +77,33 @@ internal static class RetailDetailTextureContract
     /// </summary>
     internal static bool IsNeutral(Vector4 detail, float opacity) =>
         detail.W * opacity == 0f;
+
+    /// <summary>
+    /// Review fix (post-05970306): retail's D3D fog stage runs AFTER the
+    /// texture-stage combine, applying to the FINAL pixel, not to
+    /// <c>detail.rgb</c> in isolation. acdream draws the combine as two
+    /// separate passes (mesh_modern's base draw, then mesh_detail's blended
+    /// replay), so each draw fogs its OWN colour before the fixed-function
+    /// blend recombines them — this is the CPU statement of that two-draw
+    /// path: <c>lerp(mix(base,fog,f), mix(detail,fog,f), detail.a*opacity)</c>.
+    /// It is algebraically identical to retail's single-draw
+    /// fog-after-combine order, <c>mix(Expected(base,detail,opacity), fog,
+    /// f)</c> — see <c>RetailDetailTextureContractTests</c> for the identity
+    /// pinned numerically, and mesh_detail.frag's header comment for the
+    /// derivation.
+    /// </summary>
+    internal static Vector3 ExpectedFogged(
+        Vector3 baseColour,
+        Vector4 detail,
+        float opacity,
+        Vector3 fog,
+        float fogFactor)
+    {
+        Vector3 foggedBase = Vector3.Lerp(baseColour, fog, fogFactor);
+        Vector3 foggedDetail = Vector3.Lerp(
+            new Vector3(detail.X, detail.Y, detail.Z),
+            fog,
+            fogFactor);
+        return Vector3.Lerp(foggedBase, foggedDetail, detail.W * opacity);
+    }
 }

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

@@ -4,6 +4,7 @@
 in vec2 vBaseUv;
 in vec2 vDetailUv;
 in float vDetailOpacity;
+in vec3 vWorldPos;
 in flat uint vBaseTextureIndex;
 in flat uint vBaseTextureLayer;
 in flat uint vBatchFlags;
@@ -11,6 +12,42 @@ in flat uint vDetailCategory;
 
 uniform uint uTextureIndexA; // category detail texture, layer 0
 
+// SceneLighting UBO — IDENTICAL layout to mesh_modern.frag binding=1 (same
+// std140 block, same struct, same binding). Declared here ONLY for fog
+// (uFogParams/uFogColor/uCameraAndTime); mesh_detail never lights (no
+// uLights[]/uCellAmbient read), it only needs applyFog below to match
+// retail's fog placement exactly.
+struct Light {
+    vec4 posAndKind;
+    vec4 dirAndRange;
+    vec4 colorAndIntensity;
+    vec4 coneAngleEtc;
+};
+layout(std140, ACDREAM_UBO_SET binding = 1) uniform SceneLighting {
+    Light uLights[8];
+    vec4  uCellAmbient;
+    vec4  uFogParams;
+    vec4  uFogColor;
+    vec4  uCameraAndTime;
+};
+
+// Copied verbatim from mesh_modern.frag — same math, not "improved". Retail's
+// D3D fixed-function fog stage runs AFTER the texture-stage pipeline
+// (RenderDeviceD3D's fog render state applies to the final pixel the
+// blender produced, not to an individual texture stage's output), so the
+// detail contribution must be fogged exactly like the base pass fogs its
+// own colour, with the identical fog curve.
+vec3 applyFog(vec3 lit, vec3 worldPos) {
+    int mode = int(uFogParams.w);
+    if (mode == 0) return lit;
+    float d = length(worldPos - uCameraAndTime.xyz);
+    float fogStart = uFogParams.x;
+    float fogEnd   = uFogParams.y;
+    float span = max(1e-3, fogEnd - fogStart);
+    float fog = clamp((d - fogStart) / span, 0.0, 1.0);
+    return mix(lit, uFogColor.xyz, fog);
+}
+
 out vec4 FragColor;
 
 // VM2 (2026-08-22, live cdb read on the PDB-paired retail client, GUID
@@ -37,6 +74,20 @@ out vec4 FragColor;
 // that combined alpha and lets the fixed-function blend unit do the
 // base*(1-a) + detail*a lerp. See
 // docs/research/2026-08-22-vm2-retail-detail-path-cdb.md.
+//
+// Review fix (post-05970306): retail's texture-stage combine above happens
+// BEFORE the D3D fixed-function fog stage, not after — fog is the LAST thing
+// applied to the pixel, so it applies to the lerp's result, not to detail.rgb
+// alone. mesh_modern.frag already fogs the base colour before this replay
+// draws over it (applyFog(rgb, vWorldPos) there), so fogging detail.rgb here
+// too makes the two-draw blend collapse to retail's single-draw order:
+//   (1-a)*mix(base,fog,f) + a*mix(detail,fog,f)
+//     = (1-f)*[(1-a)*base + a*detail] + f*fog
+//     = mix(lerp(base,detail,a), fog, f)
+// which is exactly retail's fog-after-combine pixel. Leaving detail.rgb
+// unfogged would draw detail at full saturation/brightness even at maximum
+// fog distance. RetailDetailTextureContractTests pins this identity
+// numerically (ExpectedFogged).
 
 void main() {
     // Object command replays may contain ordinary instances; only building
@@ -56,5 +107,5 @@ void main() {
         uTextureIndexA,
         vec3(vDetailUv, 0.0));
 
-    FragColor = vec4(detail.rgb, detail.a * vDetailOpacity);
+    FragColor = vec4(applyFog(detail.rgb, vWorldPos), detail.a * vDetailOpacity);
 }

src/AcDream.App/Rendering/Shaders/mesh_detail.vert

@@ -70,6 +70,8 @@ uniform float uParamB; // 1 = require building instance, 0 = EnvCell category
 out vec2 vBaseUv;
 out vec2 vDetailUv;
 out float vDetailOpacity;
+out vec3 vWorldPos;   // review fix: mesh_detail.frag needs this for applyFog,
+                       // exactly like mesh_modern.vert's vWorldPos.
 out flat uint vBaseTextureIndex;
 out flat uint vBaseTextureLayer;
 out flat uint vBatchFlags;
@@ -80,6 +82,7 @@ void main() {
     int instanceIndex = transformIndex - int(uTextureIndexB);
     vec4 worldPos = Instances[transformIndex].transform * vec4(aPosition, 1.0);
     gl_Position = uViewProjection * worldPos;
+    vWorldPos = worldPos.xyz;
 
     uint slot = instanceClipSlot[instanceIndex];
     CellClip clip = clipRegions[slot];

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

@@ -231,12 +231,12 @@
       "stages": [
         {
           "stage": "vert",
-          "sourceSha256": "c955bdce56199ef2057dbaf0dc1d1f75ececac49047f57685b09b63117e921da",
+          "sourceSha256": "3b03a153a439aa54fcd0bcb575274c8e1da888b8ae85ee15fae020ccb1d3bf49",
           "compiled": true
         },
         {
           "stage": "frag",
-          "sourceSha256": "c02dd48647352a87c183fe925a9590b70731c677dd942bcea71ac6ad63ff486d",
+          "sourceSha256": "e037fd28cf71453792c17522c97c4c7797193823ed78ca629a4331b632f177b1",
           "compiled": true
         }
       ]

src/AcDream.App/Rendering/Wb/EnvCellRenderer.Rhi.cs

@@ -343,10 +343,12 @@ public sealed unsafe partial class EnvCellRenderer
         }
 
         // Retail DrawEnvCell category (2). Replay the already-filtered opaque
-        // shell commands, including ClipMap built-mesh subsets, and apply the
-        // 10-50 m positive-view-depth fade. The existing
-        // "Building Detail Textures" option gates both this and buildings,
-        // matching LScape::ChangeRegion.
+        // shell commands, including ClipMap built-mesh subsets. No distance
+        // fade (VM1/VM2): retail's noFadeDetail gates get_alpha_for_z to the
+        // immediate-polygon path only, which built meshes never reach;
+        // attenuation is the sampler's linear mip chain converging to the
+        // texture mean. The existing "Building Detail Textures" option gates
+        // both this and buildings, matching LScape::ChangeRegion.
         if (renderPass == WbRenderPass.Opaque
             && detailEnabled)
         {