feat!: v4.0 breaking API cleanup + explicitApi

BREAKING changes to the core:
- treeIterator is now a read-only `val`; added `iterator(order)` and use `asSequence(order)`.
- removeChild() only removes a direct child of the receiver; added `detach()` to unhook a node.
- addChild() rejects re-parenting and cycles (throws TreeNodeException); detach() first to move.
- clear() no longer nulls the receiver's own parent; only removes descendants.
- path() returns List<TreeNode<T>>? (null) instead of throwing.

Also:
- Enable strict explicitApi() across core + both modules; add explicit `public` modifiers.
- Update tests for the new contracts + add TreeNodeV4Test; refresh .api baselines.
- README + CHANGELOG (with migration notes); bump version to 4.0.0.

47 JVM tests green.

src/commonMain/kotlin/com.github.adriankuta/datastructure/tree/iterators/LevelOrderTreeIterator.kt

@@ -20,7 +20,7 @@ import com.github.adriankuta.datastructure.tree.TreeNode
  * Output: 1 2 3 4 5 6 7 8 9 10 11 12 13
  * ```
  */
-class LevelOrderTreeIterator<T>(root: TreeNode<T>) : Iterator<TreeNode<T>> {
+public class LevelOrderTreeIterator<T>(root: TreeNode<T>) : Iterator<TreeNode<T>> {
 
     private val stack = ArrayDeque<TreeNode<T>>()
 
@@ -28,9 +28,9 @@ class LevelOrderTreeIterator<T>(root: TreeNode<T>) : Iterator<TreeNode<T>> {
         stack.addLast(root)
     }
 
-    override fun hasNext(): Boolean = stack.isNotEmpty()
+    public override fun hasNext(): Boolean = stack.isNotEmpty()
 
-    override fun next(): TreeNode<T> {
+    public override fun next(): TreeNode<T> {
         val node =  stack.removeFirst()
         node.children
             .forEach { stack.addLast(it) }

src/commonMain/kotlin/com.github.adriankuta/datastructure/tree/iterators/PostOrderTreeIterator.kt

@@ -20,7 +20,7 @@ import com.github.adriankuta.datastructure.tree.TreeNode
  * Output: 10 5 11 12 13 6 2 3 7 8 9 4 1
  * ```
  */
-class PostOrderTreeIterator<T>(root: TreeNode<T>) : Iterator<TreeNode<T>> {
+public class PostOrderTreeIterator<T>(root: TreeNode<T>) : Iterator<TreeNode<T>> {
 
     private val result = ArrayDeque<TreeNode<T>>()
 
@@ -37,7 +37,7 @@ class PostOrderTreeIterator<T>(root: TreeNode<T>) : Iterator<TreeNode<T>> {
         }
     }
 
-    override fun hasNext(): Boolean = result.isNotEmpty()
+    public override fun hasNext(): Boolean = result.isNotEmpty()
 
-    override fun next(): TreeNode<T> = result.removeFirst()
+    public override fun next(): TreeNode<T> = result.removeFirst()
 }

src/commonMain/kotlin/com.github.adriankuta/datastructure/tree/iterators/PreOrderTreeIterator.kt

@@ -20,7 +20,7 @@ import com.github.adriankuta.datastructure.tree.TreeNode
  * Output: 1 2 5 10 6 11 12 13 3 4 7 8 9
  * ```
  */
-class PreOrderTreeIterator<T>(root: TreeNode<T>) : Iterator<TreeNode<T>> {
+public class PreOrderTreeIterator<T>(root: TreeNode<T>) : Iterator<TreeNode<T>> {
 
     private val stack = ArrayDeque<TreeNode<T>>()
 
@@ -28,9 +28,9 @@ class PreOrderTreeIterator<T>(root: TreeNode<T>) : Iterator<TreeNode<T>> {
         stack.addLast(root)
     }
 
-    override fun hasNext(): Boolean = stack.isNotEmpty()
+    public override fun hasNext(): Boolean = stack.isNotEmpty()
 
-    override fun next(): TreeNode<T> {
+    public override fun next(): TreeNode<T> {
         val node = stack.removeLast()
         node.children
             .asReversed()

src/commonMain/kotlin/com.github.adriankuta/datastructure/tree/iterators/TreeNodeIterators.kt

@@ -5,7 +5,7 @@ package com.github.adriankuta.datastructure.tree.iterators
  * @see PostOrder
  * @see LevelOrder
  */
-enum class TreeNodeIterators {
+public enum class TreeNodeIterators {
     /**
      * Tree is iterated by using `Pre-order Traversal Algorithm"
      * The pre-order traversal is a topologically sorted one,