Windows: parse/verify fail with timeout wrapper (AttributeError on local function pickling)

### Summary

On Windows, `math_verify.parse` and `math_verify.verify` fail when timeout handling is enabled.
With default `raise_on_error=False`, failures are swallowed and `parse` returns `[]`, which can
silently degrade correctness checks.

### Environment

- OS: Windows 11
- Python: 3.13.3
- math-verify: 0.9.0

### Minimal Repro

```python
import math_verify

print(math_verify.parse(r"\\boxed{4}", raise_on_error=True))
print(math_verify.verify("4", "4", strict=True, timeout_seconds=1, raise_on_error=True))
```

### Observed

```text
AttributeError: Can't get local object 'timeout..decorator..wrapper..run_func'
```

When `raise_on_error=False` (default):

- `math_verify.parse(...)` returns `[]`
- `math_verify.verify(...)` returns `False`

Related symptom sometimes appears from multiprocessing teardown on Windows:

```text
OSError: [WinError 6] The handle is invalid
```

### Expected

- `parse` should return parsed candidates for valid boxed/math strings.
- `verify("4", "4", ...)` should return `True`.
- Timeout handling should work (or fail loudly with a clear platform-specific message), rather than
silently returning empty parse results / false negatives.

### Notes

The failure appears to be tied to the timeout implementation and Windows `spawn` multiprocessing,
where a local function used by the timeout wrapper is not picklable.