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/build.gradle.kts

@@ -0,0 +1,88 @@
+import org.jetbrains.kotlin.gradle.ExperimentalWasmDsl
+
+plugins {
+    alias(libs.plugins.kotlinMultiplatform)
+    alias(libs.plugins.kotlinSerialization)
+    alias(libs.plugins.dokka)
+    alias(libs.plugins.mavenPublish)
+    signing
+}
+
+group = "com.github.adriankuta"
+version = rootProject.version
+
+mavenPublishing {
+    publishToMavenCentral(automaticRelease = false)
+    signAllPublications()
+
+    coordinates("com.github.adriankuta", "tree-structure-serialization", version.toString())
+
+    pom {
+        name.set("Tree Data Structure — kotlinx.serialization")
+        description.set("kotlinx.serialization support (TreeNodeDto round-trip) for the tree-structure library.")
+        url.set("https://github.com/AdrianKuta/Tree-Data-Structure")
+        licenses {
+            license {
+                name.set("MIT License")
+                url.set("https://opensource.org/licenses/MIT")
+                distribution.set("repo")
+            }
+        }
+        developers {
+            developer {
+                id.set("AdrianKuta")
+                name.set("Adrian Kuta")
+                email.set("adrian.kuta93@gmail.com")
+            }
+        }
+        scm {
+            url.set("https://github.com/AdrianKuta/Tree-Data-Structure")
+            connection.set("scm:git:https://github.com/AdrianKuta/Tree-Data-Structure.git")
+            developerConnection.set("scm:git:ssh://git@github.com/AdrianKuta/Tree-Data-Structure.git")
+        }
+    }
+}
+
+repositories {
+    mavenCentral()
+}
+
+kotlin {
+    jvmToolchain(21)
+
+    jvm()
+
+    js(IR) {
+        browser()
+        nodejs()
+    }
+
+    @OptIn(ExperimentalWasmDsl::class)
+    wasmJs {
+        browser()
+        nodejs()
+    }
+
+    iosX64()
+    iosArm64()
+    iosSimulatorArm64()
+
+    val hostOs = System.getProperty("os.name")
+    val isMingwX64 = hostOs.startsWith("Windows")
+    when {
+        hostOs == "Mac OS X" -> macosX64("native")
+        hostOs == "Linux" -> linuxX64("native")
+        isMingwX64 -> mingwX64("native")
+        else -> throw GradleException("Host OS is not supported in Kotlin/Native.")
+    }
+
+    sourceSets {
+        commonMain.dependencies {
+            api(project(":"))
+            api(libs.kotlinx.serialization.json)
+        }
+        commonTest.dependencies {
+            implementation(kotlin("test"))
+        }
+    }
+}

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
+}

tree-structure-serialization/src/commonTest/kotlin/com.github.adriankuta/datastructure/tree/serialization/TreeNodeSerializationTest.kt

@@ -0,0 +1,40 @@
+package com.github.adriankuta.datastructure.tree.serialization
+
+import com.github.adriankuta.datastructure.tree.structurallyEquals
+import com.github.adriankuta.datastructure.tree.tree
+import kotlinx.serialization.encodeToString
+import kotlinx.serialization.json.Json
+import kotlin.test.Test
+import kotlin.test.assertEquals
+import kotlin.test.assertTrue
+
+class TreeNodeSerializationTest {
+
+    @Test
+    fun roundTripsThroughJson() {
+        val original = tree("World") {
+            child("North America") { child("USA") }
+            child("Europe") {
+                child("Poland")
+                child("Germany")
+            }
+        }
+
+        val json = Json.encodeToString(original.toDto())
+        val restored = Json.decodeFromString<TreeNodeDto<String>>(json).toTreeNode()
+
+        assertTrue(original.structurallyEquals(restored))
+    }
+
+    @Test
+    fun dtoMirrorsTreeShape() {
+        val dto = tree(1) {
+            child(2)
+            child(3) { child(4) }
+        }.toDto()
+
+        assertEquals(1, dto.value)
+        assertEquals(2, dto.children.size)
+        assertEquals(4, dto.children[1].children[0].value)
+    }
+}