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>

docs/ISSUES.md

@@ -9207,7 +9207,11 @@ the full 6,558-pass / 5-skip suite remains green.
 
 ## #226 — Retail building/EnvCell detail-texture overlay is not rendered
 
-**Status:** IMPLEMENTED + CONNECTED-VISUAL-VERIFIED 2026-08-21
+**Status:** IMPLEMENTED + CONNECTED-VISUAL-VERIFIED 2026-08-21; re-ported to
+the single-pass path 2026-08-22 by Campaign VM VM1 after VM2's live cdb read
+found retail hardware never takes the two-pass fallback this issue originally
+ported (see
+[`docs/research/2026-08-22-vm2-retail-detail-path-cdb.md`](research/2026-08-22-vm2-retail-detail-path-cdb.md)).
 **Severity:** LOW
 **Filed:** 2026-07-20
 **Component:** rendering / building and environment materials
@@ -9224,8 +9228,12 @@ mechanisms. `bb5acab9` ported the behavior that produced the observed mismatch:
 `TexMerge::CopyAndTile`/`Merge` pass each source's authored base tiling into the
 terrain composition. #226 now resolves Dereth category 1/2 detail surfaces,
 uploads their authored texture/tiling with retail wrap/linear sampling, and
-replays building and EnvCell built-mesh subsets with the exact
-`DESTCOLOR + INVSRCALPHA` blend. This includes opaque, ClipMap, straight-alpha,
+replays building and EnvCell built-mesh subsets with retail's single-pass
+detail combine (VM1/VM2, 2026-08-22): `SRCALPHA + INVSRCALPHA` compositing
+`lerp(base * diffuse, detail.rgb, detail.a * diffuseAlpha)`, matching the
+`bCanDoSinglePassDetailing = 1` path real hardware runs — not the two-pass
+`DESTCOLOR + INVSRCALPHA` fallback the port originally reproduced. This
+includes opaque, ClipMap, straight-alpha,
 additive, and inverse-alpha material subsets; transparent base/detail commands
 remain adjacent in acdream's authoritative shared alpha order with depth writes
 disabled (retail bypasses delayed alpha while detail is installed; retaining
@@ -9253,10 +9261,12 @@ its evidence source is
 
 **Acceptance:** Toggling the existing “Building Detail Textures” checkbox
 visibly changes nearby building and interior surfaces without a restart.
-Enabled detail is full through 10 m positive view depth, fades linearly to an
-exact no-op at 50 m, and preserves retail's measured slight brightening.
-Disabling it submits no detail replay and preserves the already-accepted base
-render. Landscape, ordinary objects, physics, and collision remain unchanged.
+There is no distance fade (VM1/VM2: retail's `get_alpha_for_z` is unreachable
+for built meshes; attenuation is the sampler's linear mip chain), and the
+single-pass combine darkens the live category texture by roughly 10% on
+mid-tones rather than brightening it. Disabling it submits no detail replay
+and preserves the already-accepted base render. Landscape, ordinary objects,
+physics, and collision remain unchanged.
 The automated gates cover the setting gate, data/blend/fade contract,
 built-mesh subset eligibility, opaque command filtering and A2C coverage,
 transparent depth/order seam, Vulkan descriptor