fix: avoid blocking mapping resource check

[lmaertin]

Summary

• Run the cached dedicated mapping-file existence check in a worker thread, keeping synchronous resource filesystem access off the event loop.
• Add a regression test that verifies MappingInfo.load() delegates the check through asyncio.to_thread().
• Bump the package version to 0.9.10 and document the fix.

Validation

• pytest (180 passed)
• pylint $(git ls-files '*.py') (10.00/10)
• mypy src (no issues found)
• PYTHONPATH=src python -c 'import pooldose; print(pooldose.__version__)' (0.9.10)