refactor: #100 — remove hiddenTerrainCells / BuildingTerrainCells plumbing

Retired in favour of Task 1's retail-faithful terrain shader Z nudge.
Pure removal — ~50 LOC of dead surface area across:

  - src/AcDream.Core/Terrain/LandblockMesh.cs (drop parameter +
    cell-collapse block)
  - src/AcDream.Core/World/LoadedLandblock.cs (drop field)
  - src/AcDream.Core/World/LandblockLoader.cs (drop method + call)
  - src/AcDream.App/Rendering/GameWindow.cs (3 sites)
  - src/AcDream.App/Streaming/GpuWorldState.cs (6 ctor sites)
  - src/AcDream.App/Streaming/LandblockStreamer.cs (1 ctor site)
  - tests/AcDream.Core.Tests/World/LandblockLoaderTests.cs (drop test)
  - tests/AcDream.Core.Tests/Terrain/LandblockMeshTests.cs (drop test)

No retail anchor — the deleted mechanism never had one; this commit
rolls our code back to the actual retail behaviour established in
the prior commit's shader nudge.

ISSUES.md #100 moved to Recently closed.

Cross-ref:
  docs/research/2026-05-25-issue-100-terrain-cutout-handoff.md
  docs/superpowers/plans/2026-05-25-issue-100-terrain-cutout.md

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

src/AcDream.Core/Terrain/LandblockMesh.cs

@@ -40,15 +40,13 @@ public static class LandblockMesh
     /// <param name="heightTable">Region.LandDefs.LandHeightTable — 256 float heights.</param>
     /// <param name="ctx">TerrainAtlas-derived blending inputs.</param>
     /// <param name="surfaceCache">Shared SurfaceInfo cache keyed by palette code.</param>
-    /// <param name="hiddenTerrainCells">Optional cell indices (cy * 8 + cx) to draw as zero-area triangles.</param>
     public static LandblockMeshData Build(
         LandBlock block,
         uint landblockX,
         uint landblockY,
         float[] heightTable,
         TerrainBlendingContext ctx,
-        System.Collections.Generic.IDictionary<uint, SurfaceInfo> surfaceCache,
-        System.Collections.Generic.IReadOnlySet<int>? hiddenTerrainCells = null)
+        System.Collections.Generic.IDictionary<uint, SurfaceInfo> surfaceCache)
     {
         ArgumentNullException.ThrowIfNull(block);
         ArgumentNullException.ThrowIfNull(heightTable);
@@ -168,21 +166,9 @@ public static class LandblockMesh
             }
         }
 
-        // Indices are trivial 0..383 since we don't deduplicate verts. When
-        // a building owns an outdoor terrain cell, keep the fixed 384-index
-        // contract but collapse its two triangles so the building/stair mesh
-        // can visually own the hole.
+        // Indices are trivial 0..383 since we don't deduplicate verts.
         for (uint i = 0; i < VerticesPerLandblock; i++)
-        {
-            int cellIdx = (int)i / VerticesPerCell;
-            if (hiddenTerrainCells is not null && hiddenTerrainCells.Contains(cellIdx))
-            {
-                indices[i] = (uint)(cellIdx * VerticesPerCell);
-                continue;
-            }
-
             indices[i] = i;
-        }
 
         return new LandblockMeshData(vertices, indices);
     }

src/AcDream.Core/World/LandblockLoader.cs

@@ -23,30 +23,8 @@ public static class LandblockLoader
         var entities = info is null
             ? Array.Empty<WorldEntity>()
             : BuildEntitiesFromInfo(info, landblockId);
-        var buildingTerrainCells = info is null
-            ? null
-            : BuildBuildingTerrainCells(info);
 
-        return new LoadedLandblock(landblockId, block, entities, buildingTerrainCells);
-    }
-
-    /// <summary>
-    /// Map LandBlockInfo.Buildings to 8x8 terrain mesh cells (cy * 8 + cx).
-    /// Retail attaches each CBuildingObj to its outside landcell during
-    /// CLandBlock::init_buildings; keep this signal separate from stabs so
-    /// ordinary static props do not punch holes in terrain.
-    /// </summary>
-    public static IReadOnlySet<int> BuildBuildingTerrainCells(LandBlockInfo info)
-    {
-        var result = new HashSet<int>();
-        foreach (var building in info.Buildings)
-        {
-            int cx = Math.Clamp((int)(building.Frame.Origin.X / 24f), 0, 7);
-            int cy = Math.Clamp((int)(building.Frame.Origin.Y / 24f), 0, 7);
-            result.Add(cy * 8 + cx);
-        }
-
-        return result;
+        return new LoadedLandblock(landblockId, block, entities);
     }
 
     /// <summary>

src/AcDream.Core/World/LoadedLandblock.cs

@@ -5,5 +5,4 @@ namespace AcDream.Core.World;
 public sealed record LoadedLandblock(
     uint LandblockId,
     LandBlock Heightmap,
-    IReadOnlyList<WorldEntity> Entities,
-    IReadOnlySet<int>? BuildingTerrainCells = null);
+    IReadOnlyList<WorldEntity> Entities);