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;