fix: extend action should spread a scalar string by code point

[spokodev]

action: 'extend' without nargs receives a single string value. Python's extend runs list.extend(values), and list.extend on a str iterates it by code point, so --foo abc produces ['a', 'b', 'c']. This port concatenates the string as a single element instead, producing ['abc'] — a silent divergence from the CPython 3.14.6 behaviour it targets (and not one of the intentional differences in doc/port_difference.md).

Spreading with Array.from when the value is a string matches Python's per-code-point iteration, including astral characters: --foo a😀b → ['a', '😀', 'b'], exactly as CPython 3.14.6 produces.