Add blocking read and write helpers

[iancharest]

Summary

• Add OutputStream.write_all() to wait for complete output buffer transfer.
• Add InputStream.read_exactly() to wait for fixed-size capture.
• Keep existing write() and read() methods nonblocking.
• Update play() to start the stream before blocking writes, then drain.
• Update README, architecture docs, changelog, and public API tests.

Validation

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

Closes #6.