Make Bus property public init on PixelWidgetBase

Allows host code (VkGuiRenderer) to set Bus via object initializer
on child widgets it creates (VkPlannerTab, VkEquipmentTab, etc.).

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

Author: sebgod

src/DIR.Lib/PixelWidgetBase.cs

@@ -38,7 +38,7 @@ public abstract class PixelWidgetBase<TSurface>(Renderer<TSurface> renderer) : I
         /// Optional signal bus for deferred inter-component communication.
         /// Set via object initializer at construction time.
         /// </summary>
-        protected SignalBus? Bus { get; init; }
+        public SignalBus? Bus { get; init; }
 
         /// <summary>
         /// Posts a signal to the bus for delivery at the next <see cref="SignalBus.ProcessPending"/> call.