ValueError: generator already executing when multithreading

[lilydjwg]

I run multiple threads and I get the following traceback:

      File "...", line ..., in ...:
        msg = l10n.format_value('...', {...})
      File "/usr/lib/python3.14/site-packages/fluent/runtime/fallback.py", line 41, in format_value
        for bundle in self._bundles():
      File "/usr/lib/python3.14/site-packages/fluent/runtime/fallback.py", line 61, in _bundles
        self._bundle_cache.append(next(self._bundle_it))
    ValueError: generator already executing

It seems that the iterator needs to be protected by a lock.

The version I use might be a bit old (see #219), but I checked the latest code on GitHub and the iterator is still unprotected.