Resolves the remaining Zip items from the tracking list in #54054.

The Zip iterator was missing a nth_back specialization, so calling .rev().step_by(n) (or any other pattern that ends up calling nth_back) on a zipped iterator would fall through to the default DoubleEndedIterator::nth_back, which just loops over next_back(). For TrustedRandomAccess iterators like slices, this turns what should be O(1) into O(n).

This adds nth_back at all three specialization levels in ZipImpl:

• Default (zip_impl_general_defaults macro): delegates to a super_nth_back helper that calls next_back() in a loop, same as the trait default but properly routed through the Zip dispatch.
• TrustedRandomAccessNoCoerce: inherits the default.
• TrustedRandomAccess: O(1) via direct __iterator_get_unchecked indexing, with proper length-equalization on first backward call, side-effect execution for skipped elements (as required by the MAY_HAVE_SIDE_EFFECT contract), and panic-safe self.len updates before each unchecked access.

The implementation mirrors the existing nth specialization and next_back backward-trimming logic.

Test plan

• Added test_zip_nth_back covering basic nth_back, exhaustion, and unequal-length iterators
• Added test_zip_nth_back_after_next covering interleaved forward/backward iteration
• Added test_zip_rev_nth_uses_nth_back covering the motivating use case (zip(..).rev().step_by(..))
• All existing zip tests pass (including the side-effect and panic-safety tests)

Resolves remaining items from #54054.