feat: Kotlin 2.x/K2, version catalog, serialization & coroutines modules, docs

v3.4 modernization (continued):

- Migrate to Kotlin 2.x (K2); introduce gradle/libs.versions.toml version catalog;
  simplify the JS/Wasm/Node and iOS source-set wiring for the K2 hierarchy template.
- Apply binary-compatibility-validator and Kover plugins.
- New published module tree-structure-serialization: @Serializable TreeNodeDto with
  toDto()/toTreeNode() round-trip (kotlinx.serialization).
- New published module tree-structure-coroutines: asFlow()/pre/post/levelOrderFlow()
  (kotlinx.coroutines Flow traversal).
- Docs: README examples for Sequence/navigation/functional APIs, class-level KDoc
  (thread-safety/complexity), and a CHANGELOG.md.
- Ignore subproject build/ directories.
- Bump version to 3.4.0.

All JVM tests green (core + both modules).

src/commonMain/kotlin/com.github.adriankuta/datastructure/tree/TreeNode.kt

@@ -9,7 +9,23 @@ import com.github.adriankuta.datastructure.tree.iterators.TreeNodeIterators.*
 import kotlin.jvm.JvmSynthetic
 
 /**
- * @param treeIterator Choose one of available iterators from [TreeNodeIterators]
+ * A node in a generic, mutable n-ary tree. Each node holds a [value], a reference to its [parent]
+ * and an ordered list of [children].
+ *
+ * Iterating a node (via [iterator], or the lazy [asSequence]/[preOrderSequence] extensions) visits
+ * the node and all of its descendants. Traversal and the [height]/[nodeCount]/[clear] helpers are
+ * implemented iteratively, so they are safe on arbitrarily deep trees.
+ *
+ * **Not thread-safe.** Nodes are mutable ([addChild]/[removeChild]/[clear] mutate the structure and
+ * parent pointers). Sharing a tree across threads requires external synchronization, and the tree
+ * must not be modified while it is being iterated.
+ *
+ * Equality is by reference (identity); use the `structurallyEquals` extension to compare two trees
+ * by value and shape.
+ *
+ * @param value the value stored in this node.
+ * @param treeIterator the default traversal order used by [iterator]. Prefer the
+ *   `asSequence(order)` / `preOrderSequence()` extensions to choose an order without mutating state.
  */
 open class TreeNode<T>(val value: T, var treeIterator: TreeNodeIterators = PreOrder) : Iterable<TreeNode<T>>, ChildDeclarationInterface<T> {