fix: Prev/Next responsiveness and scroll-into-view in hex search

Two fixes affecting the Prev button:

1. HexSearcher.FindBackward had an infinite loop when the search
   reached chunkStart=0 with no valid match: chunkEnd was advanced to
   `chunkStart + overlap` which on a zero start yields `overlap`, the
   same value as the previous iteration. Break out once we've
   processed the chunk anchored at offset 0.

2. HexViewControl.OnSelectionOffsetChanged only invalidated visuals
   without scrolling. The auto-scroll path was wired to
   SelectionLength only, so Next/Prev with an unchanged pattern
   length never scrolled the match into view. Route both setters
   through OnSelectionChanged.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

Author: NeWbY100

ReScene.NET/Controls/HexViewControl.cs

@@ -128,7 +128,7 @@ private static void OnSelectionOffsetChanged(DependencyObject d, DependencyPrope
         if (d is HexViewControl c)
         {
             c._canvas.ClearMouseSelection();
-            c._canvas.InvalidateVisual();
+            c.OnSelectionChanged();
         }
     }
 

ReScene.NET/Services/HexSearcher.cs

@@ -156,6 +156,13 @@ public static long FindBackward(IHexDataSource source, HexSearchPattern pattern,
                 }
             }
 
+            // When we've already scanned from the start, every preceding byte is covered;
+            // continuing would re-read the same overlap region forever.
+            if (chunkStart == 0)
+            {
+                break;
+            }
+
             chunkEnd = chunkStart + overlap;
         }