fix(core): LandblockMesh keys atlas lookup on TerrainInfo.Type

Task 1's subagent used the raw ushort as the map key because the test
used raw ushort 7 as the value. But the atlas map is built from
Region.TerrainInfo.LandSurfaces.TexMerge.TerrainDesc which keys on
TerrainTextureType enum values, extracted from bits 2-6 of the
TerrainInfo ushort per DatReaderWriter's Types/TerrainInfo.cs.

Reverts to using block.Terrain[hi].Type so the Task 2 TerrainAtlas can
actually find matching keys against real dat terrain. The test is
updated to encode Type=7 correctly as (7 << 2) in the raw ushort.

src/AcDream.Core/Terrain/LandblockMesh.cs

@@ -34,10 +34,11 @@ public static class LandblockMesh
 
                 float height = heightTable[block.Height[hi]];
 
-                // TerrainInfo raw ushort value used as the atlas-layer map key.
+                // TerrainInfo is bit-packed: bits 0-1 Road, bits 2-6 Type (5-bit
-                // The map is keyed on the raw terrain ushort (which encodes Road,
+                // TerrainTextureType enum), bits 11-15 Scenery. The atlas keys on
-                // Type, and Scenery fields), matching what the test and caller supply.
+                // Type only, matching Region.TerrainInfo.LandSurfaces.TexMerge.TerrainDesc
-                uint terrainType = (ushort)block.Terrain[hi];
+                // which lists SurfaceTexture ids per TerrainTextureType.
+                uint terrainType = (uint)block.Terrain[hi].Type;
                 if (!terrainTypeToLayer.TryGetValue(terrainType, out var layer))
                     layer = 0;
 

tests/AcDream.Core.Tests/Terrain/LandblockMeshTests.cs

@@ -89,15 +89,17 @@ public class LandblockMeshTests
     public void Build_PerVertexTerrainLayer_UsesMappedLayerIndex()
     {
         var block = BuildFlatLandBlock();
-        // TerrainInfo is a struct with implicit conversion from ushort. The low 5 bits
+        // TerrainInfo is bit-packed: bits 0-1 Road, bits 2-6 Type, bits 11-15 Scenery.
-        // of the ushort encode TerrainTextureType via TerrainInfo.Type.
+        // Raw ushort 0x001C = binary 0011100 → Type field = 7 (bits 2-6).
-        // Set vertex at x-major index (x=2, y=3) to terrain type 7.
+        // This is what a terrain sample with TerrainTextureType=7 looks like in the
-        block.Terrain[2 * 9 + 3] = (ushort)7;  // low 5 bits = 7
+        // underlying byte stream. LandblockMesh uses TerrainInfo.Type (not raw) as
+        // the atlas lookup key.
+        block.Terrain[2 * 9 + 3] = (ushort)(7 << 2);  // Type=7, Road=0, Scenery=0
 
         var map = new Dictionary<uint, uint>
         {
             [0] = 0u,   // default type → atlas layer 0
-            [7] = 4u,   // type 7 → atlas layer 4
+            [7] = 4u,   // TerrainTextureType 7 → atlas layer 4
         };
 
         var mesh = LandblockMesh.Build(block, IdentityHeightTable, map);
@@ -105,7 +107,7 @@ public class LandblockMeshTests
         // Vertex buffer internal order is y*9+x, so vertex at world (x=2, y=3) is at
         // index 3*9+2 = 29.
         Assert.Equal(4u, mesh.Vertices[3 * 9 + 2].TerrainLayer);
-        // An untouched vertex still has type 0, maps to layer 0.
+        // An untouched vertex still has Type 0, maps to layer 0.
         Assert.Equal(0u, mesh.Vertices[0].TerrainLayer);
     }