feat(data-structures): add owned binary tree with iterative traversals

[0xDevNinja]

Summary

• New src/data_structures/binary_tree.rs providing BinaryTree<T> and Node<T> with Box-owned children and an Option<Box<Node<T>>> root.
• Builders new, leaf, node; iterative inorder / preorder / postorder (explicit Vec stack), level_order (VecDeque), plus height and size (counting nodes), all returning Vec<&T> where applicable and avoiding recursion to stay safe on deep trees.
• Module-level docs cover semantics and complexity (O(n) time, O(h) / O(w) aux space); inline #[cfg(test)] mod tests covers the empty tree, single node, the classic 7-node tree (root 1, children 2/3, grandchildren 4/5/6/7) for every traversal/height/size, plus a left-skewed tree and Default.

Test plan

• cargo fmt
• cargo build
• cargo clippy --all-targets -- -D warnings
• cargo test binary_tree --lib

Closes #324