feat(app): load landblock from dats and upload mesh to GPU

GameWindow now owns a DatCollection + TerrainRenderer. On load it
opens the dat directory passed as argv[0] (or ACDREAM_DAT_DIR), finds
Holtburg (landblock 0xA9B4FFFF) by default with a fallback to the
first landblock in the cell b-tree, builds the CPU mesh from
LandblockMesh.Build, and uploads VBO+EBO+VAO with a 3f/3f/2f attribute
layout. No draw call yet — shader and matrix uniforms land in Task 9.

Enabled AllowUnsafeBlocks on the App csproj so the fixed-buffer upload
in TerrainRenderer compiles. Uses dats.Get<LandBlock>(id) instead of
TryGet(..., out T) to sidestep the [MaybeNullWhen(false)] analysis that
TreatWarningsAsErrors was flagging.

Smoke verified against the real retail dats: prints
"loaded landblock 0xA9B4FFFF" and the window stays alive with no GL
errors or exceptions.

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

src/AcDream.App/AcDream.App.csproj

@@ -7,6 +7,7 @@
     <LangVersion>latest</LangVersion>
     <TreatWarningsAsErrors>true</TreatWarningsAsErrors>
     <RootNamespace>AcDream.App</RootNamespace>
+    <AllowUnsafeBlocks>true</AllowUnsafeBlocks>
   </PropertyGroup>
   <ItemGroup>
     <PackageReference Include="Silk.NET.OpenGL" Version="2.23.0" />

src/AcDream.App/Program.cs

@@ -1,5 +1,15 @@
 using AcDream.App.Rendering;
 
-var window = new GameWindow();
+var datDir = args.FirstOrDefault()
+             ?? Environment.GetEnvironmentVariable("ACDREAM_DAT_DIR");
+
+if (string.IsNullOrWhiteSpace(datDir))
+{
+    Console.Error.WriteLine("usage: AcDream.App <dat-directory>");
+    Console.Error.WriteLine("   or: set ACDREAM_DAT_DIR and run with no args");
+    return 2;
+}
+
+using var window = new GameWindow(datDir);
 window.Run();
 return 0;

src/AcDream.App/Rendering/GameWindow.cs

@@ -1,3 +1,7 @@
+using AcDream.Core.Terrain;
+using DatReaderWriter;
+using DatReaderWriter.DBObjs;
+using DatReaderWriter.Options;
 using Silk.NET.Input;
 using Silk.NET.Maths;
 using Silk.NET.OpenGL;
@@ -7,9 +11,14 @@ namespace AcDream.App.Rendering;
 
 public sealed class GameWindow : IDisposable
 {
+    private readonly string _datDir;
     private IWindow? _window;
     private GL? _gl;
     private IInputContext? _input;
+    private TerrainRenderer? _terrain;
+    private DatCollection? _dats;
+
+    public GameWindow(string datDir) => _datDir = datDir;
 
     public void Run()
     {
@@ -45,15 +54,46 @@ public sealed class GameWindow : IDisposable
             };
 
         _gl.ClearColor(0.05f, 0.10f, 0.18f, 1.0f);
+        _gl.Enable(EnableCap.DepthTest);
+
+        _dats = new DatCollection(_datDir, DatAccessType.Read);
+
+        // Find ANY landblock ending in 0xFFFF. Holtburg 0xA9B4FFFF is a
+        // good default; fall back to the first one we find.
+        uint landblockId = 0xA9B4FFFFu;
+        var block = _dats.Get<LandBlock>(landblockId);
+        if (block is null)
+        {
+            foreach (var file in _dats.Cell.Tree)
+            {
+                if ((file.Id & 0xFFFFu) == 0xFFFFu)
+                {
+                    landblockId = file.Id;
+                    block = _dats.Get<LandBlock>(landblockId);
+                    break;
+                }
+            }
+        }
+
+        if (block is null)
+            throw new InvalidOperationException("no landblock found in cell dat");
+
+        Console.WriteLine($"loaded landblock 0x{landblockId:X8}");
+
+        var meshData = LandblockMesh.Build(block);
+        _terrain = new TerrainRenderer(_gl, meshData);
     }
 
     private void OnRender(double deltaSeconds)
     {
         _gl!.Clear(ClearBufferMask.ColorBufferBit | ClearBufferMask.DepthBufferBit);
+        _terrain?.Draw();
     }
 
     private void OnClosing()
     {
+        _terrain?.Dispose();
+        _dats?.Dispose();
         _input?.Dispose();
         _gl?.Dispose();
     }

src/AcDream.App/Rendering/TerrainRenderer.cs

@@ -0,0 +1,61 @@
+using AcDream.Core.Terrain;
+using Silk.NET.OpenGL;
+
+namespace AcDream.App.Rendering;
+
+public sealed unsafe class TerrainRenderer : IDisposable
+{
+    private readonly GL _gl;
+    private readonly uint _vao;
+    private readonly uint _vbo;
+    private readonly uint _ebo;
+    private readonly int _indexCount;
+
+    public TerrainRenderer(GL gl, LandblockMeshData meshData)
+    {
+        _gl = gl;
+        _indexCount = meshData.Indices.Length;
+
+        _vao = _gl.GenVertexArray();
+        _gl.BindVertexArray(_vao);
+
+        _vbo = _gl.GenBuffer();
+        _gl.BindBuffer(BufferTargetARB.ArrayBuffer, _vbo);
+        fixed (void* p = meshData.Vertices)
+            _gl.BufferData(BufferTargetARB.ArrayBuffer,
+                (nuint)(meshData.Vertices.Length * sizeof(Vertex)), p, BufferUsageARB.StaticDraw);
+
+        _ebo = _gl.GenBuffer();
+        _gl.BindBuffer(BufferTargetARB.ElementArrayBuffer, _ebo);
+        fixed (void* p = meshData.Indices)
+            _gl.BufferData(BufferTargetARB.ElementArrayBuffer,
+                (nuint)(meshData.Indices.Length * sizeof(uint)), p, BufferUsageARB.StaticDraw);
+
+        // vertex layout: position(3f), normal(3f), texcoord(2f) = 8 floats stride
+        uint stride = (uint)sizeof(Vertex);
+        _gl.EnableVertexAttribArray(0);
+        _gl.VertexAttribPointer(0, 3, VertexAttribPointerType.Float, false, stride, (void*)0);
+        _gl.EnableVertexAttribArray(1);
+        _gl.VertexAttribPointer(1, 3, VertexAttribPointerType.Float, false, stride, (void*)(3 * sizeof(float)));
+        _gl.EnableVertexAttribArray(2);
+        _gl.VertexAttribPointer(2, 2, VertexAttribPointerType.Float, false, stride, (void*)(6 * sizeof(float)));
+
+        _gl.BindVertexArray(0);
+    }
+
+    public void Draw()
+    {
+        // Shader binding + draw call come in Task 9. For this task, binding the
+        // VAO is enough to prove the buffers uploaded without GL errors.
+        _gl.BindVertexArray(_vao);
+        // intentionally no draw call yet
+        _gl.BindVertexArray(0);
+    }
+
+    public void Dispose()
+    {
+        _gl.DeleteBuffer(_vbo);
+        _gl.DeleteBuffer(_ebo);
+        _gl.DeleteVertexArray(_vao);
+    }
+}