fix: clear all-match highlights when no matches exist

UpdateHexMatchRanges previously assigned an empty List<HexMatchRange>
when FindAll returned nothing, which the change-tracking and DP
binding occasionally treated as a no-op vs. the prior non-empty list,
leaving stale highlights on screen. Set HexMatchRanges = null
explicitly on no-match. Also clear when the search box is emptied or
the pattern fails to parse during live typing.

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

Author: NeWbY100

ReScene.NET/ViewModels/InspectorViewModel.cs

@@ -1097,6 +1097,7 @@ private void RunLiveHexSearch()
         if (string.IsNullOrWhiteSpace(HexSearchText))
         {
             HexSearchStatus = string.Empty;
+            HexMatchRanges = null;
             return;
         }
 
@@ -1106,6 +1107,7 @@ private void RunLiveHexSearch()
         {
             // Stay quiet during typing — error message only on explicit Next/Prev.
             HexSearchStatus = string.Empty;
+            HexMatchRanges = null;
             return;
         }
 
@@ -1156,6 +1158,12 @@ private void UpdateHexMatchRanges(HexSearchPattern? pattern)
         }
 
         IReadOnlyList<long> offsets = HexSearcher.FindAll(HexDataSource, pattern);
+        if (offsets.Count == 0)
+        {
+            HexMatchRanges = null;
+            return;
+        }
+
         var ranges = new List<HexMatchRange>(offsets.Count);
         foreach (long offset in offsets)
         {