fix: replace unsafe cast with interface method in CombineWith

Replace direct cast to PruningMask<T> with GetMaskData() interface method
in the CombineWith method. This addresses the code scanning issue about
defensive casting and uses proper interface-based programming instead of
implementation-specific casting.

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

Author: franklinic

src/Pruning/PruningMask.cs

@@ -359,16 +359,20 @@ public IPruningMask<T> CombineWith(IPruningMask<T> otherMask)
             throw new ArgumentException("Masks must have same shape to combine");
 
         var combined = new Matrix<T>(_mask.Rows, _mask.Columns);
-        var otherMatrix = ((PruningMask<T>)otherMask)._mask;
 
+        // Use interface method to get mask data instead of unsafe casting
+        var otherData = otherMask.GetMaskData();
+
+        int idx = 0;
         for (int i = 0; i < _mask.Rows; i++)
         {
             for (int j = 0; j < _mask.Columns; j++)
             {
-                // Logical AND: both must be 1 to keep
+                // Logical AND: both must be non-zero to keep
                 bool keep = !_numOps.Equals(_mask[i, j], _numOps.Zero) &&
-                            !_numOps.Equals(otherMatrix[i, j], _numOps.Zero);
+                            !_numOps.Equals(otherData[idx], _numOps.Zero);
                 combined[i, j] = keep ? _numOps.One : _numOps.Zero;
+                idx++;
             }
         }