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.

tree-structure-coroutines/build.gradle.kts

@@ -47,6 +47,7 @@ repositories {
 }
 
 kotlin {
+    explicitApi()
     jvmToolchain(21)
 
     jvm()