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).