Add PostmanSort algorithm

- Implements Postman Sort using gap-based insertion approach
- Adaptive gap reduction with shrink factor of 1.3
- Includes comprehensive test suite extending SortingAlgorithmTest

Author: PauLopNun

src/main/java/com/thealgorithms/sorts/PostmanSort.java

@@ -0,0 +1,45 @@
+package com.thealgorithms.sorts;
+
+import java.util.Arrays;
+
+/**
+ * Postman Sort Algorithm Implementation
+ *
+ * @see <a href="https://en.wikipedia.org/wiki/Postman_sort">Postman Sort Algorithm</a>
+ */
+public class PostmanSort implements SortAlgorithm {
+
+    @Override
+    public <T extends Comparable<T>> T[] sort(T[] array) {
+        if (array == null || array.length <= 1) {
+            return array;
+        }
+        postmanSort(array);
+        return array;
+    }
+
+    private <T extends Comparable<T>> void postmanSort(T[] array) {
+        int n = array.length;
+        double gapFactor = 1.3;
+        int gap = (int) (n / gapFactor);
+
+        while (gap > 0) {
+            for (int i = gap; i < n; i++) {
+                T temp = array[i];
+                int j = i;
+                while (j >= gap && SortUtils.greater(array[j - gap], temp)) {
+                    array[j] = array[j - gap];
+                    j -= gap;
+                }
+                array[j] = temp;
+            }
+            if (gap == 1) {
+                break;
+            }
+            gap = (int) (gap / gapFactor);
+            if (gap < 1) {
+                gap = 1;
+            }
+        }
+    }
+}

src/test/java/com/thealgorithms/sorts/PostmanSortTest.java

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+package com.thealgorithms.sorts;
+
+public class PostmanSortTest extends SortingAlgorithmTest {
+    @Override
+    SortAlgorithm getSortAlgorithm() {
+        return new PostmanSort();
+    }
+}