Phase 2: Arrow Integration and Efficient Data Transfer

[wdconinc]

• Implement pqxx::result to arrow::Table conversion in C++.
• Expose Arrow tables to Python with zero-copy transfer.
• Convenience Pandas/Polars converters.
• Add chunked streaming API for large queries.
• Index recommendations and performance tests.

Deliverable: Efficient zero-copy data transfer for large results.