feat: stack-safe traversal + lazy Sequence + navigation/functional extensions

Core additive work for v3.4 (non-breaking):

- Rewrite nodeCount(), height(), clear() and the post-order iterator iteratively
  so deep/degenerate trees no longer throw StackOverflowError (verified to 50k deep).
- Add lazy Sequence traversal: asSequence(order), pre/post/levelOrderSequence().
- Add navigation extensions: isLeaf, degree, root(), ancestors(), siblings(),
  leaves(), descendants().
- Add functional extensions: findNode, filterNodes, anyNode, allNodes, countNodes,
  foldNodes, mapValues, deepCopy, structurallyEquals (all stack-safe).
- Add tests for stack-safety, the new APIs, and previously-uncovered
  height/depth/nodeCount/path (incl. exception paths). 40 tests green on JVM.

src/commonTest/kotlin/com.github.adriankuta/datastructure.tree/TreeNodeUtilitiesTest.kt

@@ -0,0 +1,54 @@
+package com.github.adriankuta.datastructure.tree
+
+import com.github.adriankuta.datastructure.tree.exceptions.TreeNodeException
+import kotlin.test.Test
+import kotlin.test.assertContentEquals
+import kotlin.test.assertEquals
+import kotlin.test.assertFailsWith
+
+class TreeNodeUtilitiesTest {
+
+    private val root = TreeNode(1)
+    private val a = TreeNode(2)
+    private val b = TreeNode(3)
+
+    init {
+        root.addChild(a)
+        a.addChild(b)
+    }
+
+    @Test
+    fun nodeCountCountsDescendantsExcludingRoot() {
+        assertEquals(0, TreeNode("solo").nodeCount())
+        assertEquals(2, root.nodeCount())
+    }
+
+    @Test
+    fun heightIsLongestEdgePathToLeaf() {
+        assertEquals(0, TreeNode("solo").height())
+        assertEquals(2, root.height())
+        assertEquals(1, a.height())
+    }
+
+    @Test
+    fun depthIsDistanceToRoot() {
+        assertEquals(0, root.depth())
+        assertEquals(1, a.depth())
+        assertEquals(2, b.depth())
+    }
+
+    @Test
+    fun pathReturnsDescendantToReceiverChain() {
+        assertContentEquals(listOf(b, a, root), root.path(b))
+    }
+
+    @Test
+    fun pathThrowsWhenNotADescendant() {
+        assertFailsWith<TreeNodeException> { root.path(TreeNode(99)) }
+    }
+
+    @Test
+    fun pathThrowsWhenDescendantIsRootItself() {
+        assertFailsWith<TreeNodeException> { root.path(root) }
+    }
+}