Add mirrored control bytes to enable seamless wraparound in probing

- Introduced mirrored control bytes at the end of the control byte array to allow
  seamless wraparound during group probing.
- This mirrors the first 16 bytes (GroupSize) of the control array at the end,
  enabling efficient SIMD-based probing across boundaries without additional
  boundary checks or wraparound logic.
- This design improvement allows continuous group probing by treating the control
  byte array as circular, thereby enhancing performance, reducing branching, and
  simplifying the probe sequence.

src/DenseMap.cs

@@ -394,16 +394,13 @@ public bool Get(TKey key, out TValue value)
             // Compare the target vector (hashed key) with the loaded source vector to find matches.
             // `ExtractMostSignificantBits()` returns a mask where each bit set indicates a match.
             var mask = Vector128.Equals(target, source).ExtractMostSignificantBits();
-
             // Process any matches indicated by the mask.
             while (mask != 0)
             {
                 // Get the position of the first set bit in the mask (indicating a match).
                 var bitPos = BitOperations.TrailingZeroCount(mask);
-
                 // Retrieve the entry corresponding to the matched bit position within the map's entries.
                 var entry = Find(_entries, index + Unsafe.As<int, byte>(ref bitPos));
-
                 // Check if the entry's key matches the specified key using the equality comparer.
                 if (_comparer.Equals(entry.Key, key))
                 {
@@ -516,7 +513,7 @@ public ref TValue GetValueRefOrAddDefault(TKey key)
                 // Set the key for the located entry to the specified `key`.
                 entry.Key = key;
                 // Set the control byte for the entry at position `i` to `h2` to mark it as occupied.
-                Find(_controlBytes, i) = h2;
+                SetCtrl(i, h2);
                 // Increment the total count of entries in the hash table.
                 Count++;
 
@@ -615,7 +612,6 @@ public bool Update(TKey key, TValue value)
         }
     }
 
-
     /// <summary>
     /// Removes a key and value from the map.
     /// Example:
@@ -920,7 +916,7 @@ private void Resize()
                     var bitPos = BitOperations.TrailingZeroCount(mask);
                     index += Unsafe.As<int, uint>(ref bitPos);
 
-                    Find(_controlBytes, index) = h2;
+                    SetCtrl(i, h2);
                     Find(_entries, index) = entry;
                     break;
                 }