docs(vm1): closeout - AP-232 for the translucent detail blend weight; sampler test pins the production constant

Opus narrow re-review of ae651312: APPROVE. Closes its three residuals:
- AP-232 filed: retail's single-pass stage-1 OUTPUT alpha
  (MODULATE(TEXTURE, CURRENT) @0x0059c549) is the blend weight for a
  translucent subset; acdream's two-draw model is exact for opaque
  subsets (fog identity pinned) and a bounded weight difference on
  translucent ones. Distinct from AP-34 (queue order). Owed since
  05970306.
- TerrainAtlas.DetailSamplerDescription names the production sampler
  (WRAP/LINEAR x3 per ACRender::SetDetailSurfaceInternal @0x006b6280);
  the test now asserts that constant's properties instead of a
  test-local copy.
- Plan VM1 section: fragment now described as fogged; VM1 marked CLOSED
  with the Holtburg measurement (+2.17/+0.57/+0.16 vs predicted
  +2.2/+0.66/+0.16) and the detail-on cost (+0.3-0.5 ms CPU at Arwic).

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

src/AcDream.App/Rendering/TerrainAtlas.cs

@@ -34,6 +34,16 @@ public sealed class TerrainAtlas : IDisposable
     /// one-layer 2-D array so it uses the same backend-neutral table contract
     /// as every other world texture.
     /// </summary>
+    /// <summary>
+    /// Retail's detail-stage sampler: <c>ACRender::SetDetailSurfaceInternal</c>
+    /// (0x006b6280) sets WRAP/WRAP addressing and LINEAR/LINEAR/LINEAR filtering
+    /// on the detail stage. The LINEAR mip chain is retail's only distance
+    /// attenuation for built meshes (VM1), so this constant is load-bearing and
+    /// pinned by <c>TerrainAtlasDetailTextureTests</c>.
+    /// </summary>
+    internal static readonly GpuSamplerDescription DetailSamplerDescription =
+        GpuSamplerDescription.WorldRepeat;
+
     internal readonly record struct RetailDetailTextureBinding(
         GpuTextureSlot TextureSlot,
         float Tiling,
@@ -430,7 +440,7 @@ public sealed class TerrainAtlas : IDisposable
             // ChangeRegion and the only reachable SmartBox caller keep
             // landscape (0) and object (3) disabled, so do not create or expose
             // those categories here.
-            detailSampler = device.CreateSampler(GpuSamplerDescription.WorldRepeat);
+            detailSampler = device.CreateSampler(DetailSamplerDescription);
             if (terrainDesc is { Count: > 1 })
             {
                 buildingDetail = TryCreateDetailTexture(