Save Muse 2/S PPG and IMU as sidecar CSVs

[khuzaymahbinharis-jpg]

Summary

• Save Muse 2 / Muse S PPG and accel/gyro from BrainFlow as sidecar CSVs next to the existing EEG recording (recording_...._ppg.csv, recording_...._accel.csv).
• Enable PPG with p51 (Muse 2) / p61 (Muse S) so the EEG file keeps four channels and the same sampling rate. Extra streams are not concatenated, because EEG (~256 Hz), accel (~52 Hz), and PPG (~64 Hz) do not share a rate.
• BlueMuse / MuseLSL and Muse 2016 are out of scope. I do not have a Muse headset; tests mock BrainFlow presets. A maintainer with a device should confirm a real recording.

Fixes #233

Test plan

• python -m pytest tests/test_acquisition.py -o addopts= (synthetic acquisition plus mocked Muse sidecar tests)
• Record with muse2_bfn or museS_bfn on a real headset and confirm three CSVs: EEG (unchanged columns), _ppg.csv, _accel.csv
• Confirm passing config= still overrides the default Muse command
• Confirm non-Muse boards (e.g. synthetic) still write only the EEG CSV