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).

build.gradle.kts

@@ -1,19 +1,17 @@
-import org.jetbrains.kotlin.gradle.targets.js.dsl.ExperimentalWasmDsl
-import org.jetbrains.kotlin.gradle.targets.js.dsl.KotlinWasmTargetDsl
-import org.jetbrains.kotlin.gradle.targets.js.ir.KotlinJsIrTarget
-import org.jetbrains.kotlin.gradle.targets.js.nodejs.NodeJsRootExtension
-import org.jetbrains.kotlin.gradle.targets.js.nodejs.NodeJsRootPlugin
+import org.jetbrains.kotlin.gradle.ExperimentalWasmDsl
 
 plugins {
-    kotlin("multiplatform") version "1.9.24"
-    id("org.jetbrains.dokka") version "1.9.20"
-    id("com.vanniktech.maven.publish") version "0.34.0"
+    alias(libs.plugins.kotlinMultiplatform)
+    alias(libs.plugins.dokka)
+    alias(libs.plugins.mavenPublish)
+    alias(libs.plugins.binaryCompatibilityValidator)
+    alias(libs.plugins.kover)
     signing
 }
 
 val PUBLISH_GROUP_ID = "com.github.adriankuta"
 val PUBLISH_ARTIFACT_ID = "tree-structure"    // base artifact; KMP will add -jvm, -ios*, etc.
-val PUBLISH_VERSION = "3.1.5"
+val PUBLISH_VERSION = "3.4.0"
 
 val snapshot: String? by project
 
@@ -29,7 +27,10 @@ mavenPublishing {
 
     pom {
         name.set("Tree Data Structure")
-        description.set("Simple implementation to store object in tree structure.")
+        description.set(
+            "Lightweight n-ary tree data structure for Kotlin Multiplatform (JVM, JS, Wasm, iOS, " +
+                "Native). DSL, pre/post/level-order traversal, lazy Sequence traversal, and pretty-print.",
+        )
         url.set("https://github.com/AdrianKuta/Tree-Data-Structure")
 
         licenses {
@@ -54,28 +55,15 @@ mavenPublishing {
     }
 }
 
-// No legacy publishing {} block or s01 repos — Central Portal handles it.
-
 repositories {
     mavenCentral()
 }
 
-java {
-    toolchain {
-        languageVersion.set(JavaLanguageVersion.of(21))
-    }
-}
 kotlin {
-    jvmToolchain(21);
-    jvm {
-        compilations.all {
-            kotlinOptions {
-                jvmTarget = "21"   // <- was "1.8"
-            }
-        }
-    }
+    jvmToolchain(21)
+
+    jvm()
 
-    // JS targets (IR) for publishing
     js(IR) {
         browser()
         nodejs()
@@ -87,21 +75,7 @@ kotlin {
         nodejs()
     }
 
-    rootProject.plugins.withType<NodeJsRootPlugin> {
-        rootProject.extensions.getByType<NodeJsRootExtension>().nodeVersion = "22.0.0"
-    }
-
-    kotlin.targets.withType<KotlinJsIrTarget> {
-        if (name == "wasmJs") {
-            @Suppress("UNCHECKED_CAST")
-            (this as KotlinWasmTargetDsl).apply {
-                nodejs {
-                }
-            }
-        }
-    }
-
-    // iOS targets
+    // Apple targets
     iosX64()
     iosArm64()
     iosSimulatorArm64()
@@ -109,7 +83,7 @@ kotlin {
     // Native host target
     val hostOs = System.getProperty("os.name")
     val isMingwX64 = hostOs.startsWith("Windows")
-    val nativeTarget = when {
+    when {
         hostOs == "Mac OS X" -> macosX64("native")
         hostOs == "Linux" -> linuxX64("native")
         isMingwX64 -> mingwX64("native")
@@ -117,25 +91,8 @@ kotlin {
     }
 
     sourceSets {
-        val commonMain by getting
-        val commonTest by getting { dependencies { implementation(kotlin("test")) } }
-        val jvmMain by getting
-        val jvmTest by getting
-        val jsMain by getting
-        val jsTest by getting
-        val wasmJsMain by getting
-        val wasmJsTest by getting
-        val nativeMain by getting
-        val nativeTest by getting
-
-        // Shared iOS source sets
-        val iosMain by creating { dependsOn(commonMain) }
-        val iosTest by creating { dependsOn(commonTest) }
-        val iosX64Main by getting { dependsOn(iosMain) }
-        val iosArm64Main by getting { dependsOn(iosMain) }
-        val iosSimulatorArm64Main by getting { dependsOn(iosMain) }
-        val iosX64Test by getting { dependsOn(iosTest) }
-        val iosArm64Test by getting { dependsOn(iosTest) }
-        val iosSimulatorArm64Test by getting { dependsOn(iosTest) }
+        commonTest.dependencies {
+            implementation(kotlin("test"))
+        }
     }
 }