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-coroutines/src/commonMain/kotlin/com.github.adriankuta/datastructure/tree/coroutines/TreeNodeFlowExt.kt

@@ -0,0 +1,24 @@
+package com.github.adriankuta.datastructure.tree.coroutines
+
+import com.github.adriankuta.datastructure.tree.TreeNode
+import com.github.adriankuta.datastructure.tree.asSequence
+import com.github.adriankuta.datastructure.tree.iterators.TreeNodeIterators
+import kotlinx.coroutines.flow.Flow
+import kotlinx.coroutines.flow.asFlow
+
+/**
+ * Emits this node and all of its descendants as a cold [Flow], traversed in the given [order].
+ * Useful for plugging tree traversal into coroutine/Flow pipelines (e.g. in a ViewModel).
+ */
+public fun <T> TreeNode<T>.asFlow(
+    order: TreeNodeIterators = TreeNodeIterators.PreOrder,
+): Flow<TreeNode<T>> = asSequence(order).asFlow()
+
+/** Pre-order traversal as a cold [Flow]. */
+public fun <T> TreeNode<T>.preOrderFlow(): Flow<TreeNode<T>> = asFlow(TreeNodeIterators.PreOrder)
+
+/** Post-order traversal as a cold [Flow]. */
+public fun <T> TreeNode<T>.postOrderFlow(): Flow<TreeNode<T>> = asFlow(TreeNodeIterators.PostOrder)
+
+/** Level-order (breadth-first) traversal as a cold [Flow]. */
+public fun <T> TreeNode<T>.levelOrderFlow(): Flow<TreeNode<T>> = asFlow(TreeNodeIterators.LevelOrder)

tree-structure-coroutines/src/commonTest/kotlin/com.github.adriankuta/datastructure/tree/coroutines/TreeNodeFlowTest.kt

@@ -0,0 +1,35 @@
+package com.github.adriankuta.datastructure.tree.coroutines
+
+import com.github.adriankuta.datastructure.tree.iterators.TreeNodeIterators
+import com.github.adriankuta.datastructure.tree.tree
+import kotlinx.coroutines.flow.map
+import kotlinx.coroutines.flow.toList
+import kotlinx.coroutines.test.runTest
+import kotlin.test.Test
+import kotlin.test.assertEquals
+
+class TreeNodeFlowTest {
+
+    private fun sample() = tree(1) {
+        child(2) {
+            child(4)
+            child(5)
+        }
+        child(3) {
+            child(6)
+        }
+    }
+
+    @Test
+    fun preOrderFlowEmitsInPreOrder() = runTest {
+        assertEquals(listOf(1, 2, 4, 5, 3, 6), sample().preOrderFlow().map { it.value }.toList())
+    }
+
+    @Test
+    fun levelOrderFlowEmitsInLevelOrder() = runTest {
+        assertEquals(
+            listOf(1, 2, 3, 4, 5, 6),
+            sample().asFlow(TreeNodeIterators.LevelOrder).map { it.value }.toList(),
+        )
+    }
+}