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" />