feat: add runnable :samples module (#47)

* build: scaffold :samples module (issue #37)

* feat: add runnable, verified examples to :samples (issue #37)

* ci,docs: run :samples in CI and document ./gradlew :samples:run (issue #37)

samples/build.gradle.kts

@@ -0,0 +1,35 @@
+plugins {
+    // No version: the Kotlin Gradle plugin is already on the build classpath via the root
+    // project's kotlinMultiplatform plugin, so requesting a version here would clash.
+    kotlin("jvm")
+    application
+}
+
+kotlin {
+    jvmToolchain(21)
+}
+
+application {
+    mainClass.set("com.github.adriankuta.samples.SamplesKt")
+}
+
+repositories {
+    mavenCentral()
+}
+
+dependencies {
+    implementation(project(":"))
+    implementation(project(":tree-structure-serialization"))
+    implementation(project(":tree-structure-coroutines"))
+    implementation(project(":tree-structure-immutable"))
+    // ImmutableTreeNode.children returns a PersistentList, so consumers that touch it need
+    // kotlinx.collections.immutable on their own classpath (the module declares it as implementation).
+    implementation(libs.kotlinx.collections.immutable)
+
+    testImplementation(kotlin("test"))
+}
+
+// kotlin("test") auto-selects the JUnit 5 adapter when the test task uses the JUnit Platform.
+tasks.test {
+    useJUnitPlatform()
+}