feat: add Traversable utilities

[ghoullier]

Summary

Add Traversable utilities that enable inverting nested structures like Array<Option<T>> into Option<Array<T>> for fail-fast collection processing.

What's included

• traverse — maps each element through a function returning Option, collecting results into Option<R[]> (returns None if any element fails)
• sequence — specialization of traverse with identity, flipping Option<T>[] into Option<T[]>
• traverseResult — same concept for Result: returns Ok of array if all succeed, or the first Error
• sequenceResult — flips Result<T, E>[] into Result<T[], E>

Why

Traversable is a fundamental FP pattern for working with collections of effectful computations. Instead of manually iterating and checking each result, these utilities provide a declarative way to "fail fast" when processing arrays where each element may succeed or fail.