Add O2Ring-S OxyII support

[saim256]

Summary

• add an OxyII protocol frame codec and O2Ring-S/T8520 downloader state machine
• detect O2Ring-S sync-mode advertisements by OxyII service UUID, S8-AW/T8520 names, and manufacturer data
• download O2Ring-S recordings through the OxyII file-transfer flow and convert the v1 recording format to CSV
• document the protocol source used for the implementation

Fixes #5.

Notes

This is based on the public protocol writeup and reference implementation from https://github.com/nglessner/o2ring-s-protocol. I do not have physical O2Ring-S hardware attached in this environment, so the live BLE path still needs validation by someone with a T8520/O2Ring-S. The local checks cover syntax, OxyII frame encoding/decoding, file-list parsing, auth/time payload shape, and synthetic v1 recording conversion.

Verification

• python -m compileall o2r o2ring.py o2convert.py
• focused Python fixture for OxyII frame codec/file-list parsing/auth payload shape/synthetic v1 recording conversion
• git diff --check