Fix thread safety issues

[kmnhan]

I've found that trying to load files with a different byte order in different threads leads to a race condition and fails, due to the PackedFileRecordHeader object being global.

The following code is a MWE that reproduces the race condition:

import threading
from igor2.packed import load


def worker(filename, thread_id):
    expt = None
    for bo in ('<', '>'):
        try:
            _, expt = load(filename, initial_byte_order=bo)
        except ValueError:
            pass
    if expt is None:
        raise ValueError("No experiment loaded")


threads = []
for i, fname in enumerate(["tests/data/packed-byteorder.pxt"] * 100):
    t = threading.Thread(target=worker, args=(fname, i))
    threads.append(t)
    t.start()

for t in threads:
    t.join()

This PR ensures thread safety by creating a new instance of the structure for each load instead of reusing the singleton.

[paulmueller]

@kmnhan Thank you!
For some reason GH Actions is not shown here. Actions fail due to flake8: https://github.com/AFM-analysis/igor2/actions/runs/14465886033

Could you please fix those, then I will merge 🙏

[kmnhan]

I've fixed it in aad6c00, could you try running the workflow again? Thanks!

[paulmueller]

Right, sorry. I'm just going to merge now. No idea why GH Actions did not run..