feat(core): add LandblockMesh flat-terrain generator

Pure CPU mesh generator: takes a DatReaderWriter LandBlock DBObj and
produces 81 vertices + 128 triangles covering 192x192 world units.
Vertices are a readonly record struct (position, normal, texcoord)
so the upcoming GPU upload in Task 8 can sizeof() them directly.
Height byte -> world z uses a simple 2x scale; the real AC height
lookup table is a Phase 2+ concern.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

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

@@ -0,0 +1,76 @@
+using System.Numerics;
+using AcDream.Core.Terrain;
+using DatReaderWriter.DBObjs;
+using DatReaderWriter.Types;
+
+namespace AcDream.Core.Tests.Terrain;
+
+public class LandblockMeshTests
+{
+    private static LandBlock BuildFlatLandBlock(byte heightIndex = 0)
+    {
+        var block = new LandBlock
+        {
+            HasObjects = false,
+            Terrain = new TerrainInfo[81],
+            Height = new byte[81],
+        };
+        for (int i = 0; i < 81; i++)
+        {
+            block.Terrain[i] = (ushort)0;
+            block.Height[i] = heightIndex;
+        }
+        return block;
+    }
+
+    [Fact]
+    public void Build_FlatBlock_Produces81VerticesAnd128Triangles()
+    {
+        var block = BuildFlatLandBlock();
+
+        var mesh = LandblockMesh.Build(block);
+
+        Assert.Equal(81, mesh.Vertices.Length);
+        Assert.Equal(128 * 3, mesh.Indices.Length);
+    }
+
+    [Fact]
+    public void Build_Vertices_Cover192x192WorldUnits()
+    {
+        var block = BuildFlatLandBlock();
+
+        var mesh = LandblockMesh.Build(block);
+
+        var minX = mesh.Vertices.Min(v => v.Position.X);
+        var maxX = mesh.Vertices.Max(v => v.Position.X);
+        var minY = mesh.Vertices.Min(v => v.Position.Y);
+        var maxY = mesh.Vertices.Max(v => v.Position.Y);
+
+        Assert.Equal(0.0f, minX);
+        Assert.Equal(192.0f, maxX);
+        Assert.Equal(0.0f, minY);
+        Assert.Equal(192.0f, maxY);
+    }
+
+    [Fact]
+    public void Build_FlatBlock_AllVerticesSameZ()
+    {
+        var block = BuildFlatLandBlock(heightIndex: 10);
+
+        var mesh = LandblockMesh.Build(block);
+
+        var zs = mesh.Vertices.Select(v => v.Position.Z).Distinct().ToArray();
+        Assert.Single(zs);
+    }
+
+    [Fact]
+    public void Build_HeightValues_ScaleByTwo()
+    {
+        var block = BuildFlatLandBlock(heightIndex: 5);
+
+        var mesh = LandblockMesh.Build(block);
+
+        // AC's Land::LandHeightTable scales height byte index by 2.0f for the simple ramp case.
+        Assert.Equal(10.0f, mesh.Vertices[0].Position.Z);
+    }
+}