fix(render): #226 detail overlay uses retail's single-pass combine; drop the dead distance fade (Campaign VM VM1)

VM2's live cdb read against the PDB-paired retail client (GUID
9e847e2f-777c-4bd9-886c-22256bb87f32) proved
m_caps.bCanDoSinglePassDetailing = 1 and trysinglepass = 1 on real hardware,
so D3DPolyRender::RenderMeshSubset (0x0059ca10) never falls back to the
two-pass framebuffer blend the earlier #226 port reproduced. Every loaded
CGfxObj sets use_built_mesh = 1 (CGfxObj::InitLoad 0x005346b0), so buildings
and EnvCells always take the single-pass texture-stage combine set up in
D3DPolyRender::SetSurface (0x0059c4d0):

    result = lerp(base * diffuse, detail.rgb, detail.a * diffuse.a)

RenderMeshSubset lights opaque built-mesh subsets with
tmpmaterial.Diffuse.a = 1, so on the live Dereth category texture
0x06006D58 (mean rgb 0.165, mean alpha 0.132) the combine works out to
~0.868 * base + 0.022 — a mild darkening, the opposite sign of the fallback
DstColor blend's brightening.

Also removes the invented 10 m / 50 m distance fade. Retail's
ACRender::get_alpha_for_z (0x006b6230) is only evaluated in
D3DPolyRender::DrawPolyInternal (0x0059d7c0, the immediate-polygon path)
and only when the static noFadeDetail (0x00820e38, initialised to 1) is 0 —
unreachable for built meshes. Attenuation is the sampler's linear mip chain
converging to the texture mean, not a scripted ramp.

Changes:
- mesh_detail.vert/.frag: drop vDetailFade and its distance term; add
  vDetailOpacity mirroring mesh_modern.vert's InstanceAlphaBuf (binding 7)
  read, and output detail.rgb with alpha = detail.a * vDetailOpacity under
  the corrected pipeline blend.
- VulkanViewportMapping.BlendFactorsOf / GpuEnums.GpuBlendMode.RetailDetail:
  SrcAlpha + OneMinusSrcAlpha instead of DstColor + OneMinusSrcAlpha.
- RetailDetailTextureContract: replaced the distance-fade constants and
  FramebufferFactor with Expected(base, detail, opacity) and IsNeutral,
  matching the lerp; contract tests cover zero-alpha/zero-opacity no-ops,
  the measured darkening on the live category texture, and full-alpha
  replacement.
- Regenerated mesh_detail's committed SPIR-V and the shader manifest
  (tools/compile-shaders.ps1); no other shader pair changed.
- Docs: #226's pseudocode note, the docs/ISSUES.md #226 entry, and the
  retired TS-52 divergence-register row corrected from the two-pass
  DESTCOLOR description to the single-pass path and the darkening
  expectation, each citing the VM2 cdb note.

Verified: dotnet build AcDream.slnx -c Release (0 warnings, 0 errors);
dotnet test on AcDream.App.Tests and AcDream.Core.Tests (Release, hermetic
lanes) both green.

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

src/AcDream.App/Rendering/Gpu/GpuEnums.cs

@@ -137,10 +137,13 @@ internal enum GpuBlendMode
     InverseAlpha,
 
     /// <summary>
-    /// Retail building/EnvCell detail overlay:
-    /// <c>DstColor, OneMinusSrcAlpha</c>. This intentionally preserves the
-    /// retail client's measured brightening; it is not a conventional
-    /// modulate/roughening blend.
+    /// Retail building/EnvCell detail overlay: <c>SrcAlpha,
+    /// OneMinusSrcAlpha</c>. VM2's live cdb read proved retail hardware runs
+    /// the single-pass texture-stage combine (<c>D3DPolyRender::SetSurface</c>
+    /// 0x0059c4d0), a lerp toward the detail colour by
+    /// <c>detail.a * diffuse.a</c> — a mild darkening on the live Dereth
+    /// category texture, not the two-pass <c>DstColor</c> fallback's
+    /// brightening.
     /// </summary>
     RetailDetail,
 }

src/AcDream.App/Rendering/Gpu/Vk/VulkanViewportMapping.cs

@@ -175,9 +175,12 @@ internal static class VulkanViewportMapping
         GpuBlendMode.Additive => (BlendFactor.SrcAlpha, BlendFactor.One),
         // Retail's third mode, found at slice V4c in WbDrawDispatcher.ApplyRetailBlend.
         GpuBlendMode.InverseAlpha => (BlendFactor.OneMinusSrcAlpha, BlendFactor.SrcAlpha),
-        // ACRender::SetDetailSurfaceInternal with DrawBuilding/DrawEnvCell's
-        // category state: D3DBLEND_DESTCOLOR + D3DBLEND_INVSRCALPHA.
-        GpuBlendMode.RetailDetail => (BlendFactor.DstColor, BlendFactor.OneMinusSrcAlpha),
+        // VM2 (live cdb read, 2026-08-22): real hardware runs the single-pass
+        // texture-stage detail combine, not the two-pass DESTCOLOR fallback.
+        // D3DPolyRender::SetSurface's stage-1 BLENDCURRENTALPHA is a lerp
+        // toward the detail colour by detail.a * diffuse.a, which is the
+        // ordinary straight-alpha-over factor pair.
+        GpuBlendMode.RetailDetail => (BlendFactor.SrcAlpha, BlendFactor.OneMinusSrcAlpha),
         GpuBlendMode.None => (BlendFactor.One, BlendFactor.Zero),
         _ => throw new ArgumentOutOfRangeException(nameof(blend), blend, "Unknown blend mode."),
     };