Add Baichuan fallback for GetOsd/SetOsd

[andrewvenson]

Summary

• Adds a Baichuan implementation of GetOsd/SetOsd so Host.set_osd() also works on devices that are only reachable over the Baichuan protocol (bc_only=True, i.e. no HTTP(s) API available).
• GetOsd now decodes the Baichuan XML response into the same _osd_settings[channel] shape the HTTP JSON API produces, so Host.set_osd() itself needed no changes — it works transparently regardless of which protocol is in use underneath.
• Adds a SetOsd Baichuan command (cmd_id 45) that fetches the current OSD XML (cmd_id 44), patches only the fields being changed, and writes it back.

Position encoding

The HTTP JSON API expresses OSD position as a string ("Upper Left", "Bottom Center", etc, see validate_osd_pos), but Baichuan expresses it as raw topLeftX/topLeftY integers. There's no official documentation for this encoding, so I derived it empirically by cycling through all 6 HTTP positions and reading back the resulting Baichuan XML values:

Position	topLeftX	topLeftY
Upper Left	1	1
Upper Right	65536	1
Top Center	65537	1
Bottom Center	65537	65536
Lower Left	1	65536
Lower Right	65536	65536

Confirmed identical across two different device types:

• Reolink E1 Pro (firmware v3.1.0.4417_2412122130)
• Reolink Video Doorbell WiFi (firmware v3.0.0.4662_2508071282)

Usage

No API changes required — existing code works unmodified, including on bc_only hosts:

from reolink_aio.api import Host

host = Host(ip, username, password, bc_only=True)  # works the same with bc_only=False
await host.get_host_data()
await host.set_osd(0, namePos="Upper Left", datePos="Lower Right", enableWaterMark=True)

Testing

• Verified end-to-end against both devices above with bc_only=True: enable/disable, name text, position, and watermark toggle all round-trip correctly and were cross-checked against the raw Baichuan XML readback.
• black, isort, flake8, and pylint (10.00/10) all pass on the changed file; mypy reports no issues.

[starkillerOG]

Why do you need this fallback?

[andrewvenson]

Why do you need this fallback?

The hope is to limit the amount of attack vectors on my network. I would rather use the baicuan protocol to update settings on my cameras rather than temporarily opening up http/s ports to make changes.

Sometimes after power outages the osd settings are erased. I have a cron that targets this branch to set the osd on the cams on a daily basis.