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/Log.cs

@@ -0,0 +1,45 @@
+namespace MrGameEng.Core;
+
+/// <summary>Severity of a log message.</summary>
+public enum LogLevel
+{
+    /// <summary>Verbose diagnostics.</summary>
+    Debug,
+
+    /// <summary>Normal informational message.</summary>
+    Info,
+
+    /// <summary>Something suspicious but recoverable.</summary>
+    Warning,
+
+    /// <summary>An error.</summary>
+    Error,
+}
+
+/// <summary>
+/// Engine-wide logger. Messages go to subscribers (the developer console subscribes here)
+/// and to the debugger output. Logging allocates — do not log every frame from hot paths.
+/// </summary>
+public static class Log
+{
+    /// <summary>Raised for every message. May be invoked from any thread.</summary>
+    public static event Action<LogLevel, string>? MessageLogged;
+
+    /// <summary>Logs a debug message.</summary>
+    public static void Debug(string message) => Write(LogLevel.Debug, message);
+
+    /// <summary>Logs an informational message.</summary>
+    public static void Info(string message) => Write(LogLevel.Info, message);
+
+    /// <summary>Logs a warning.</summary>
+    public static void Warning(string message) => Write(LogLevel.Warning, message);
+
+    /// <summary>Logs an error.</summary>
+    public static void Error(string message) => Write(LogLevel.Error, message);
+
+    private static void Write(LogLevel level, string message)
+    {
+        System.Diagnostics.Debug.WriteLine($"[{level}] {message}");
+        MessageLogged?.Invoke(level, message);
+    }
+}

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) =>