Add in-game developer console (MrGameEng.DevConsole)

Core gains a static Log (Debug/Info/Warning/Error + event); the engine
logs key events like scene switches. The console captures Log output
into a 2048-line ring buffer and executes registered commands with
input history (up/down), Tab prefix completion and scrolling
(PageUp/PageDown/End/wheel). Built-ins: help, clear, echo, timescale,
close, quit; games register their own (sample: stress/main/beep).

Console core is pure logic covered by headless tests; the Myra overlay
renders a single label rebuilt only when the Revision counter moves -
an idle or closed console costs nothing per frame. Toggled with the
backquote key; sample gameplay hotkeys are suppressed while open.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

src/MrGameEng.Core/SceneManager.cs

@@ -111,6 +111,7 @@ public sealed class SceneManager
         Current = _pending;
         _pending = null;
         Current?.Load(_context);
+        Log.Info($"Scene switched to {Current?.GetType().Name ?? "<none>"}");
     }
 
     private static float Advance(float coverage, float direction, float duration, float deltaTime) =>