[Perf] parallel block-existence checks + timeout exception

[mag1c-h]

Summary

1. Parallelize block-existence checks to cut latency on bulk operations.
2. Add timeout guards that raise a Python exception when the deadline is exceeded.

Details

• Issue: sequential existence checks become a bottleneck when the caller asks for hundreds/thousands of blocks.
• Fix: dispatch all Lookup calls to a thread-pool and wait for the batch with a configurable timeout.
• Behavior change: if any single check or the whole batch exceeds timeout_ms, a pybind11::timeout_error is propagated to the Python side.