fix: correctness bugs flagged by CodeRabbit

Four pre-existing upstream defects fixed in v1.0.0:

- Util/GlobalParametersCache.cs GetValue captures Cache into a local
  before the bounds check, so the check and the indexed read operate
  on the same array reference even when Refresh reassigns Cache from
  the main thread between the two operations
- Util/IconUtil.cs binary search bounds: hi initialized to
  entries.Length-1 (was Length), and reset on redirect-restart;
  added entries.Length==0 short-circuit to prevent indexing into
  empty arrays
- Sheets.cs WorldsOnDatacenter compared Region.RowId, which groups
  by region instead of datacenter — now compares DataCenter.RowId
  directly so the result actually reflects same-DC worlds
- Message.cs back-reference loop iterates the processed Sender/Content
  properties rather than the raw constructor parameters, so chunks
  added or replaced by CheckMessageContent also get Message set

HellionChat/Util/GlobalParametersCache.cs

@@ -10,10 +10,14 @@ public static class GlobalParametersCache
 
     public static int GetValue(int index)
     {
-        if (index < 0 || index >= Cache.Length)
+        // Capture the array reference once so the bounds check and the
+        // indexed read operate on the same instance, even if Refresh
+        // reassigns Cache between the two operations.
+        var cache = Cache;
+        if (index < 0 || index >= cache.Length)
             return 0;
 
-        return Cache[index];
+        return cache[index];
     }
 
     /// <summary>

HellionChat/Util/IconUtil.cs

@@ -59,8 +59,14 @@ public readonly unsafe ref struct GfdFileView
             return false;
         }
 
+        if (entries.Length == 0)
+        {
+            entry = default;
+            return false;
+        }
+
         var lo = 0;
-        var hi = entries.Length;
+        var hi = entries.Length - 1;
         while (lo <= hi)
         {
             var i = lo + ((hi - lo) >> 1);
@@ -70,7 +76,7 @@ public readonly unsafe ref struct GfdFileView
                 {
                     iconId = entries[i].Redirect;
                     lo = 0;
-                    hi = entries.Length;
+                    hi = entries.Length - 1;
                     continue;
                 }