Add native async/await support

[aezell]

Summary

• add AsyncSpritesClient with native async lifecycle, filesystem, command, checkpoint, service, session, and network-policy operations
• share command state and pure path behavior between sync and async implementations
• scope multiplexed control pools by client and event loop, close them from aclose(), and prune closed-loop entries
• preserve and test the completed exit status on synchronous Cmd instances

User impact

Async applications such as FastAPI services can use sprites-py without blocking their event loop or routing calls through asyncio.to_thread(). Existing synchronous APIs remain available and behavior-compatible, with Cmd.exit_code now correctly reporting the completed status.

Closes #23.

Verification

• python -m ruff check src tests
• python -m ruff format --check src tests
• python -m mypy src
• python -m pytest -q (118 passed)
• python -m build

Deployment

No service deployment is required. This is a library-only change and can ship through the normal sprites-py PyPI release process.