docs: adding subject and main

subjects/java/checkpoints/harmonious-fusion/ExerciseRunner.java

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+import java.util.Arrays;
+
+public class ExerciseRunner {
+    public static void main(String[] args) {
+        HarmoniousFusion merger = new HarmoniousFusion();
+
+        // Test case 1
+        int[] arr1 = {1, 3, 5};
+        int[] arr2 = {2, 4, 6};
+        int[] mergedArray = merger.merge(arr1, arr2);
+        System.out.println(Arrays.toString(mergedArray)); // Expected output: [1, 2, 3, 4, 5, 6]
+
+        // Test case 2
+        int[] arr3 = {1, 3, 5, 7};
+        int[] arr4 = {2, 4, 6};
+        mergedArray = merger.merge(arr3, arr4);
+        System.out.println(Arrays.toString(mergedArray)); // Expected output: [1, 2, 3, 4, 5, 6, 7]
+    }
+}

subjects/java/checkpoints/harmonious-fusion/README.md

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+## Harmonious Fusion
+### Instructions
+
+Create a class `HarmoniousFusion` that provides a method to merge two sorted arrays into one sorted array. The merged array should also be sorted.
+
+### Expected Class
+
+```java
+public class HarmoniousFusion {
+    public int[] merge(int[] arr1, int[] arr2) {
+        // Implementation to merge two sorted arrays into one sorted array
+    }
+}
+```
+
+### Usage
+
+Here is a possible `ExerciseRunner.java` to test your class:
+
+```java
+import java.util.Arrays;
+
+public class ExerciseRunner {
+    public static void main(String[] args) {
+        HarmoniousFusion merger = new HarmoniousFusion();
+
+        // Test case 1
+        int[] arr1 = {1, 3, 5};
+        int[] arr2 = {2, 4, 6};
+        int[] mergedArray = merger.merge(arr1, arr2);
+        System.out.println(Arrays.toString(mergedArray)); // Expected output: [1, 2, 3, 4, 5, 6]
+
+        // Test case 2
+        int[] arr3 = {1, 3, 5, 7};
+        int[] arr4 = {2, 4, 6};
+        mergedArray = merger.merge(arr3, arr4);
+        System.out.println(Arrays.toString(mergedArray)); // Expected output: [1, 2, 3, 4, 5, 6, 7]
+    }
+}
+```
+
+### Expected Output
+
+```shell
+$ javac *.java -d build
+$ java -cp build ExerciseRunner
+[1, 2, 3, 4, 5, 6]
+[1, 2, 3, 4, 5, 6, 7]
+$
+```
+