Feature/centroid decomposition

[lens161]

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Summary: This PR adds an implementation of centroid decomposition for trees. The algorithm recursively finds centroids of subtrees and builds a centroid tree with height O(log n). This structure is useful for answering distance, nearest node, and other divide and conquer tree queries.

Details:

Supports adding edges to an undirected tree

Builds the centroid decomposition via build()

Provides access to centroid parents and children

Includes a helper to iterate over centroid ancestors

Tests cover subtree sizes, parent relationships, children, reset behaviour, and consistency