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/TreeNodeExt.kt

@@ -3,7 +3,7 @@ package com.github.adriankuta.datastructure.tree
 import com.github.adriankuta.datastructure.tree.iterators.TreeNodeIterators
 import kotlin.jvm.JvmSynthetic
 
-typealias ChildDeclaration<T> = ChildDeclarationInterface<T>.() -> Unit
+public typealias ChildDeclaration<T> = ChildDeclarationInterface<T>.() -> Unit
 
 /**
  * This method can be used to initialize new tree.
@@ -14,7 +14,7 @@ typealias ChildDeclaration<T> = ChildDeclarationInterface<T>.() -> Unit
  * @see [ChildDeclarationInterface.child]
  */
 @JvmSynthetic
-inline fun <reified T> tree(
+public inline fun <reified T> tree(
     root: T,
     defaultIterator: TreeNodeIterators = TreeNodeIterators.PreOrder,
     childDeclaration: ChildDeclaration<T>