feat(app): Silk.NET window smoke — clear to navy

Minimal Silk.NET window wiring: 1280x720, OpenGL 4.3 core profile,
VSync, dark navy clear color, Escape to close. No rendering beyond
the clear call — terrain and shader land in Tasks 8 and 9.

Manual smoke: process starts, stays alive past GL context creation,
produces no stderr, no uncaught exceptions. Actual visual check
will happen end-to-end after Task 10.

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

src/AcDream.App/Program.cs

@@ -1,3 +1,5 @@
-// src/AcDream.App/Program.cs
-Console.WriteLine("acdream app — phase 1 skeleton");
+using AcDream.App.Rendering;
+
+var window = new GameWindow();
+window.Run();
 return 0;

src/AcDream.App/Rendering/GameWindow.cs

@@ -0,0 +1,62 @@
+using Silk.NET.Input;
+using Silk.NET.Maths;
+using Silk.NET.OpenGL;
+using Silk.NET.Windowing;
+
+namespace AcDream.App.Rendering;
+
+public sealed class GameWindow : IDisposable
+{
+    private IWindow? _window;
+    private GL? _gl;
+    private IInputContext? _input;
+
+    public void Run()
+    {
+        var options = WindowOptions.Default with
+        {
+            Size = new Vector2D<int>(1280, 720),
+            Title = "acdream — phase 1",
+            API = new GraphicsAPI(
+                ContextAPI.OpenGL,
+                ContextProfile.Core,
+                ContextFlags.ForwardCompatible,
+                new APIVersion(4, 3)),
+            VSync = true,
+        };
+
+        _window = Window.Create(options);
+        _window.Load += OnLoad;
+        _window.Render += OnRender;
+        _window.Closing += OnClosing;
+
+        _window.Run();
+    }
+
+    private void OnLoad()
+    {
+        _gl = GL.GetApi(_window!);
+        _input = _window!.CreateInput();
+        foreach (var kb in _input.Keyboards)
+            kb.KeyDown += (_, key, _) =>
+            {
+                if (key == Key.Escape)
+                    _window!.Close();
+            };
+
+        _gl.ClearColor(0.05f, 0.10f, 0.18f, 1.0f);
+    }
+
+    private void OnRender(double deltaSeconds)
+    {
+        _gl!.Clear(ClearBufferMask.ColorBufferBit | ClearBufferMask.DepthBufferBit);
+    }
+
+    private void OnClosing()
+    {
+        _input?.Dispose();
+        _gl?.Dispose();
+    }
+
+    public void Dispose() => _window?.Dispose();
+}