Add tests to verify default worker behavior in ParallelMap and
ParallelFilter.
Add internal test to cover unexported Result marker methods for
coverage.
- Add `Chunk` to split slices into groups of size n.
- Add `ContainsBy` for predicate-based containment checks.
- Add `UniqueBy` to deduplicate slices by key selector.
- Add `ParallelMap` for concurrent mapping with context and error
handling.
- Add `maps.Keys` and `maps.Values` helpers for extracting map
keys/values.
- Update README and docs for new features.
- Refactor `Contains` to use `slices.Contains`.
