Refactor TreeCenter: simplify algorithm, add docs and tests

[williamfiset]

Summary

• Added class-level Javadoc with algorithm description ("peeling leaf layers"), O(V+E) / O(V) complexity
• Simplified findTreeCenters() — removed intermediate variables and redundant degree[node] = 0
• Removed @author Modifications by line
• Replaced LinkedList with ArrayList, cleaned up imports
• Trimmed main to one significant example with ASCII diagram
• Added starGraph() and asymmetricTree() tests to test file
• Cleaned up wildcard imports in test file

Test plan

• All 9 TreeCenterTest tests pass (including randomized cross-impl check)

🤖 Generated with Claude Code