Add a Fenwick tree data structure

[Suvanwita]

Summary

Add a Fenwick tree (Binary Indexed Tree) implementation to src/data_structures.

Requirements

• Support creating an empty Fenwick tree for a given size.
• Support point updates and prefix sum queries.
• Support range sum queries via prefix sum combination.
• Support efficient initialization from an array.
• Support retrieving the total sum of all elements.
• Document the indexing convention and zero-based or one-based behavior clearly.
• Handle invalid indices consistently and document the behavior.

Testing

Add tests covering:

• empty and single-element trees
• point updates
• prefix sum queries
• range sum queries
• initialization from an array
• invalid index updates and queries

Documentation

Document the public API and include a short usage example consistent with the repository’s existing data structure modules.