feat(data-structures): add Fenwick tree

[0xDevNinja]

Summary

Adds a Fenwick (binary-indexed) tree over i64 with point update, prefix sum, and inclusive range sum.

Closes #13.

Implementation notes

• Internally 1-indexed using i & -i trick for parent traversal (wrapping_neg to satisfy clippy on usize).
• O(log n) per operation, O(n) space.

Test plan

• Empty
• Single element with multiple updates
• 8-element prefix sum after sequential updates
• range_sum with [lo, hi] inclusive bounds
• Negative deltas (subtraction support)
• fmt / clippy / cargo test green

[0xDevNinja]

Reviewed

• Implementation: i64 Fenwick tree with point update + prefix sum + range sum
• Tests: 5 cases including negative deltas
• Complexity: O(log n) per op, O(n) space
• fmt / clippy / cargo test: green

Merging via rebase.