Add native macOS duplex stream

[iancharest]

Summary

• Add _native.DuplexStream on macOS.
• Implement one native duplex object that owns Core Audio input and output queues together.
• Add Linux miniaudio duplex support using one native ma_device_type_duplex callback.
• Wire NativeBackend.open_duplex_stream() to native handles on Core Audio and miniaudio.
• Convert native duplex stats into DuplexStreamStats.
• Update README and architecture docs now that Linux duplex is implemented.

Validation

• PYTHONPATH=src python3 -m unittest discover -s tests
• python3 -m compileall -q src tests examples
• Isolated Linux wheel build: python3 -m pip wheel . --no-deps -w /tmp/tachyaudio-wheel
• Wheel-installed Linux null-device duplex smoke test: create, write silence, start, read 128 input frames, stats, close.
• Real Linux PipeWire/PulseAudio explicit-device duplex smoke test: create, write silence, start, read 256 input frames, stats, close.
• Previous local Core Audio duplex smoke test: create, write silence, start, read 64 input frames, stats, close.

Continues #4.