fix: _cut_iter yields wrong-length parts when step > n

partition()/chunks() on an iterator (_cut_iter) only correctly skipped
step items between parts when step <= n. For step > n, slicing an
n-length pool by pool[step:] silently clips to an empty list, then
islice(it, step) pulled step (not n) fresh items into the pool instead
of first discarding the step - n items between parts. This produced
wrong-length, wrong-content chunks that diverged from the Sequence
code path (_cut_seq) for the same input, e.g. list(range(10)) vs
iter(range(10)) with n=2, step=3.

Branch on step < n to keep the existing sliding-window logic, and for
step >= n explicitly discard the gap before taking the next pool.

funcy/seqs.py

@@ -363,8 +363,12 @@ def _cut_iter(drop_tail, n, step, seq):
         if len(pool) < n:
             break
         yield pool
-        pool = pool[step:]
-        pool.extend(islice(it, step))
+        if step < n:
+            pool = pool[step:]
+            pool.extend(islice(it, step))
+        else:
+            take(step - n, it)  # discard the gap between parts
+            pool = take(n, it)
     if not drop_tail:
         for item in _cut_seq(drop_tail, n, step, pool):
             yield item