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>

src/AcDream.Core/Terrain/LandblockMesh.cs

@@ -0,0 +1,50 @@
+using System.Numerics;
+using DatReaderWriter.DBObjs;
+
+namespace AcDream.Core.Terrain;
+
+public sealed record LandblockMeshData(Vertex[] Vertices, uint[] Indices);
+
+public static class LandblockMesh
+{
+    // AC landblock geometry constants
+    private const int VerticesPerSide = 9;                // 9x9 heightmap grid
+    private const int CellsPerSide = VerticesPerSide - 1; // 8x8 cells
+    private const float CellSize = 24.0f;                 // world units per cell edge
+    private const float HeightScale = 2.0f;               // byte height -> world z
+
+    public static LandblockMeshData Build(LandBlock block)
+    {
+        var vertices = new Vertex[VerticesPerSide * VerticesPerSide];
+        for (int y = 0; y < VerticesPerSide; y++)
+        {
+            for (int x = 0; x < VerticesPerSide; x++)
+            {
+                int i = y * VerticesPerSide + x;
+                float height = block.Height[i] * HeightScale;
+                vertices[i] = new Vertex(
+                    Position: new Vector3(x * CellSize, y * CellSize, height),
+                    Normal: Vector3.UnitZ,
+                    TexCoord: new Vector2(x / (float)CellsPerSide, y / (float)CellsPerSide));
+            }
+        }
+
+        var indices = new uint[CellsPerSide * CellsPerSide * 6];
+        int idx = 0;
+        for (int y = 0; y < CellsPerSide; y++)
+        {
+            for (int x = 0; x < CellsPerSide; x++)
+            {
+                uint a = (uint)(y * VerticesPerSide + x);
+                uint b = (uint)(y * VerticesPerSide + x + 1);
+                uint c = (uint)((y + 1) * VerticesPerSide + x);
+                uint d = (uint)((y + 1) * VerticesPerSide + x + 1);
+                // two triangles per cell, CCW
+                indices[idx++] = a; indices[idx++] = b; indices[idx++] = d;
+                indices[idx++] = a; indices[idx++] = d; indices[idx++] = c;
+            }
+        }
+
+        return new LandblockMeshData(vertices, indices);
+    }
+}

src/AcDream.Core/Terrain/Vertex.cs

@@ -0,0 +1,5 @@
+using System.Numerics;
+
+namespace AcDream.Core.Terrain;
+
+public readonly record struct Vertex(Vector3 Position, Vector3 Normal, Vector2 TexCoord);