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).

tree-structure-serialization/src/commonMain/kotlin/com.github.adriankuta/datastructure/tree/serialization/TreeNodeDto.kt

@@ -0,0 +1,30 @@
+package com.github.adriankuta.datastructure.tree.serialization
+
+import com.github.adriankuta.datastructure.tree.TreeNode
+import kotlinx.serialization.Serializable
+
+/**
+ * A serializable, acyclic view of a [TreeNode] subtree. [TreeNode] itself holds a back-reference to
+ * its parent (a cycle), so it cannot be `@Serializable` directly — convert to/from this DTO instead.
+ *
+ * ```
+ * val json = Json.encodeToString(tree.toDto())
+ * val restored = Json.decodeFromString<TreeNodeDto<String>>(json).toTreeNode()
+ * ```
+ */
+@Serializable
+public data class TreeNodeDto<T>(
+    val value: T,
+    val children: List<TreeNodeDto<T>> = emptyList(),
+)
+
+/** Converts this subtree into a serializable [TreeNodeDto], preserving values and shape. */
+public fun <T> TreeNode<T>.toDto(): TreeNodeDto<T> =
+    TreeNodeDto(value, children.map { it.toDto() })
+
+/** Rebuilds a mutable [TreeNode] tree from this DTO. */
+public fun <T> TreeNodeDto<T>.toTreeNode(): TreeNode<T> {
+    val node = TreeNode(value)
+    children.forEach { node.addChild(it.toTreeNode()) }
+    return node
+}