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.

README.md

@@ -104,9 +104,8 @@ World
 val root = TreeNode("root")
 // ... build your tree
 
-// Choose iteration order (default is PreOrder)
-root.treeIterator = TreeNodeIterators.PostOrder
-for (node in root) println(node.value)
+// Choose iteration order per call (the default order is set in the constructor and is read-only)
+for (node in root.asSequence(TreeNodeIterators.PostOrder)) println(node.value)
 
 // Utilities
 root.nodeCount()   // number of descendants
@@ -114,10 +113,10 @@ root.height()      // longest path to a leaf (in edges)
 root.depth()       // distance from current node to the root
 val path = root.path(root.children.first()) // nodes from descendant up to root
 
-// Mutations
+// Mutations — removeChild removes a *direct* child; detach() unhooks a node from wherever it lives
 val child = root.children.first()
-root.removeChild(child)
-root.clear()       // remove entire subtree
+root.removeChild(child)   // child is now detached from root
+root.clear()              // remove all descendants of root
 ```
 
 ### Lazy traversal with Sequence