retiff needs to do its own globbing for file arguments on Windows

Nobody has claimed this yet.

Apparently, the Windows shell (`cmd.exe`, at least) does not do things like filename globbing before setting `argv` in spawned processes. (There is something like this available through the MSVC runtime helpers, but the Python interpreter does not opt in to that.)

To get convenient behavior that is more consistent across platforms, we can add our own globber for the positional arguments to `retiff`. Something like the following.

```python
import argparse
import glob

class GlobFiles(argparse.Action):
def __call__(self, parser, namespace, values, option_string=None):
out = []
for v in values:
matches = glob.glob(v)
if matches:
out.extend(matches)
else:
out.append(v) # keep literal (bash-like default)
setattr(namespace, self.dest, out)

p = argparse.ArgumentParser()
p.add_argument(
"infile",
nargs="+",
action=GlobFiles,
)
```

We ought to add GitHub Actions testing pipelines and include Windows testing before committing a change and resolving this issue, though.

Start at retiff's positional-argument parsing and compare the proposed argparse Action with the current handling of infile values. Add Windows coverage through a GitHub Actions testing pipeline, preserving unmatched literals, and confirm the pipeline exercises filename globbing before resolving the issue.

Written by the indexing model from the issue text.