C extension: pickling any FrozenList raises TypeError (atomic[bint] not convertible)

**Version**: current master (`frozenlist 1.8.1.dev0`, commit after the free-threading changes)
**Platform**: Windows, CPython 3.13.7 — not platform-specific

### Summary

Any `FrozenList` instance fails to pickle when the C extension is used:

```python
import pickle
from frozenlist import FrozenList

fl = FrozenList([3, 1, 2])
fl.freeze()
pickle.dumps(fl)
```

```
TypeError: self._frozen cannot be converted to a Python object for pickling
```

The pure-Python implementation (`FROZENLIST_NO_EXTENSIONS=1`) round-trips fine on the same interpreter, so the two implementations have diverged.

### Root cause

`_frozenlist.pyx` declares `cdef atomic[bint] _frozen` (added for free-threading support). Cython's auto-generated `__reduce_cython__` cannot convert a `libcpp.atomic.atomic[bint]` member to a Python object, so pickling raises for every instance.

There is also no test coverage for pickling at all right now, which is why this went unnoticed.

### Suggested fix

An explicit `__reduce__` in the pyx class that serializes `(items, frozen)` through a small module-level constructor restores parity with the pure-Python implementation without giving up the atomic.