Add duplex stream API contract

[iancharest]

Summary

• Add DuplexStreamConfig, DuplexStreamStats, and public DuplexStream.
• Add backend-level open_duplex_stream() instead of composing separate input/output streams.
• Reuse blocking read/write helper internals for duplex streams.
• Document the backend-level synchronization requirement.
• Mark native duplex support as not implemented yet.

Validation

• PYTHONPATH=src python3 -m unittest discover -s tests
• python3 -m compileall -q src tests examples

Note: local python3 -m mypy src was not run because mypy is not installed in the active Python environment.

Starts #4.