fix(arrays): stop fill from growing the array on reversed spans

fill rebuilt the array as array[:start] + fill + array[end:]. When start
came after end those two slices overlapped, duplicating the shared
elements and growing the array while writing no value. Normalize start and
end into [0, length] and fill in place with an index loop so an empty span
is a no-op and the length stays fixed.

tests/test_arrays.py

@@ -158,6 +158,14 @@ def test_duplicates(case, expected):
         (([1, 2, 3, 4, 5], 0, 0, 5), [0, 0, 0, 0, 0]),
         (([1, 2, 3, 4, 5], 0, 0, 8), [0, 0, 0, 0, 0]),
         (([1, 2, 3, 4, 5], 0, 0, -1), [0, 0, 0, 0, 5]),
+        # start after end must be a no-op and must not grow the array.
+        (([1, 2, 3, 4], "x", 4, 1), [1, 2, 3, 4]),
+        (([1, 2, 3, 4, 5], 0, 3, 2), [1, 2, 3, 4, 5]),
+        # negative start supported the same way slicing does.
+        (([1, 2, 3, 4, 5], 0, -2), [1, 2, 3, 0, 0]),
+        (([1, 2, 3, 4, 5], 0, -2, -1), [1, 2, 3, 0, 5]),
+        # reversed negative span is a no-op, not an array-growing overlap.
+        (([1, 2, 3], "x", -1, -2), [1, 2, 3]),
     ],
 )
 def test_fill(case, expected):