Add Fenwick tree (binary indexed tree)

[0xDevNinja]

Point update / prefix sum query in O(log n). Generic over a Group trait or specialised on i64.

Acceptance criteria

• Implementation lives in src/<category>/<algorithm>.rs.
• Inline #[cfg(test)] mod tests { ... } covering empty, single-element, and representative inputs.
• Property test (where feasible) comparing against a reference implementation (e.g. slice::sort).
• Doc comment on the public function names the algorithm and its complexity.
• cargo fmt --check, cargo clippy --all-targets -- -D warnings, and cargo test all pass.