Offload synchronous filesystem I/O to asyncio.to_thread in LocalFSBackend

[mchades]

Context

LocalFSBackend.execute() is async but performs synchronous filesystem I/O (iterdir, stat, read_bytes, write_bytes, mkdir) which blocks the event loop.

Scope

All four intent types are affected:

• LOOKUP: read_bytes()
• QUERY: iterdir(), stat()
• INGEST: mkdir(), write_bytes(), write_text()
• REVISE: read_bytes(), write_bytes()

Proposed Approach

Wrap blocking calls with asyncio.to_thread() or use aiofiles for file I/O.

References

• PR [#46] feat(backends): add BLOB_STORAGE backend with local filesystem provider #47 review comment by @jerryshao