Problem:
Iter:peek() only works if the iterator is a |list-iterator| (internally, an `ArrayIter`).
However, it is possible to implement :peek() support for any iterator.
Solution:
- add `_peeked` buffer for lookahead without actually consuming values
- `peek()` now works for function, pairs(), and array iterators
- `skip(predicate)` stops at the first non matching element without consuming it
- keep existing optimized behavior for `ArrayIter` to maintain backward compatibility
- use `pack`/`unpack` to support iterators that return multiple values
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