Add Ch18 Kani harnesses for slice iterator functions

39 total proof harnesses covering:
- Iter (existing 18 harnesses across 4 types)
- IterMut: new, into_slice, as_mut_slice, next, next_back
- Split: next, next_back
- Chunks: next, next_back, __iterator_get_unchecked
- ChunksMut: next, nth, next_back, nth_back
- ChunksExact: new
- ChunksExactMut: next, nth, next_back, nth_back
- RChunks: next, next_back
- RChunksMut: next, nth, last, next_back, nth_back
- RChunksExact: new
- RChunksExactMut: next, nth, next_back, nth_back
- ArrayWindows: next/next_back (N=1,2,4), nth/nth_back (N=2,4)
- Windows: __iterator_get_unchecked

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

Author: jrey8343