Refactor RootingTree with docs, final fields, and diagrams

[williamfiset]

Summary

• Added clear class-level Javadoc with algorithm description and O(V+E) time / O(V) space complexity
• Made id, parent, children fields final in TreeNode
• Replaced LinkedList with ArrayList for better cache locality
• Replaced wildcard import java.util.* with explicit imports
• Added Javadoc to rootTree() and buildTree() methods
• Added ASCII diagram of the undirected tree structure before rooting examples

Test plan

• bazel build //src/main/java/.../treealgorithms:treealgorithms compiles successfully
• Review output of main() matches expected layer comments

🤖 Generated with Claude Code