feat(app): slow default fly speed and add Shift-boost

The user noted that the previous 35 u/s default felt too fast for
exploring scenery — overshooting buildings and skipping past entities
constantly. Drop default to 12 u/s (≈AC's run speed), and bind Shift
to a 50 u/s boost for fast travel between landblocks. Backwards
compatible: the new boost parameter on FlyCamera.Update has a default
of false, so any existing caller behaves the same.

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

src/AcDream.App/Rendering/GameWindow.cs

@@ -1003,7 +1003,8 @@ public sealed class GameWindow : IDisposable
             s: kb.IsKeyPressed(Key.S),
             d: kb.IsKeyPressed(Key.D),
             up: kb.IsKeyPressed(Key.Space),
-            down: kb.IsKeyPressed(Key.ControlLeft));
+            down: kb.IsKeyPressed(Key.ControlLeft),
+            boost: kb.IsKeyPressed(Key.ShiftLeft) || kb.IsKeyPressed(Key.ShiftRight));
     }
 
     private void OnCameraModeChanged(bool isFlyMode)