feat: add async wrapper as extension module

[jgwesterlund]

Pull Request

What does this change do?

Adds AsyncSDSavior in sdsavior.aio as a separate extension module wrapping SDSavior for asyncio applications. Uses a dedicated single-thread executor and asyncio.Lock for thread safety. Implements chunked async iteration to avoid list materialization.

Why is this needed?

Enables using SDSavior in async applications without blocking the event loop. Per reviewer feedback, this is a separate extension module (not integrated into core __init__.py) — users import explicitly from sdsavior.aio.

Key design decisions addressing reviewer bugs:

• Thread safety: Uses ThreadPoolExecutor(max_workers=1) by default (not run_in_executor with the default pool which runs parallel threads)
• No list materialization: iter_records is an AsyncIterator that yields records in chunks of 256, avoiding memory spikes for large rings

Type of change

• Feature

How was this tested?

8 new tests in tests/test_async.py:

• Basic append and iterate
• Context manager
• Export to JSONL
• Flush
• Concurrent appends are serialized (thread-safety verification)
• Custom executor
• Reopen persistence
• from_seq filtering

All 44 tests pass, 98% coverage, ruff clean, mypy clean.

Checklist

• I tested this locally
• I didn't reformat the entire repo
• I wrote sane commit messages
• I read the contributing guide
• This probably won't summon new bugs

Notes for reviewers

AsyncSDSavior is NOT added to __init__.py.__all__ — it's opt-in via from sdsavior.aio import AsyncSDSavior.

Added pytest-asyncio as a dev dependency and asyncio_mode = "auto" to pyproject.toml.

Docs updated: usage.md, api.md.