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/commonTest/kotlin/com.github.adriankuta/datastructure.tree/TreeNodeUtilitiesTest.kt

@@ -1,10 +1,9 @@
 package com.github.adriankuta.datastructure.tree
 
-import com.github.adriankuta.datastructure.tree.exceptions.TreeNodeException
 import kotlin.test.Test
 import kotlin.test.assertContentEquals
 import kotlin.test.assertEquals
-import kotlin.test.assertFailsWith
+import kotlin.test.assertNull
 
 class TreeNodeUtilitiesTest {
 
@@ -43,12 +42,12 @@ class TreeNodeUtilitiesTest {
     }
 
     @Test
-    fun pathThrowsWhenNotADescendant() {
-        assertFailsWith<TreeNodeException> { root.path(TreeNode(99)) }
+    fun pathReturnsNullWhenNotADescendant() {
+        assertNull(root.path(TreeNode(99)))
     }
 
     @Test
-    fun pathThrowsWhenDescendantIsRootItself() {
-        assertFailsWith<TreeNodeException> { root.path(root) }
+    fun pathReturnsNullWhenDescendantIsRootItself() {
+        assertNull(root.path(root))
     }
 }