feat(cyton): per-channel config, AUX analog mode, buffer drain, FTDI latency check

[pellet]

Configuration and recording options for the OpenBCI Cyton.

• eegnb.devices.cyton: cyton_ch_config() builds per-channel gain and BIAS settings from the GAIN_* codes, and thinkpulse_config() does the same for ThinkPulse active electrodes. Channel commands are retried up to 3 times when the board doesn't reply Success, which is usually a dropped serial reply.
• EEG(analog_mode=True) switches the Cyton to analog mode (/2) and records the AUX pins as AUX0… columns, for any analog sensor such as a photodiode.
• EEG.drain() writes the recording so far. BlockExperiment calls it after each block, so the pattern-reversal VEP keeps completed blocks if a session crashes, and long sessions stay clear of BrainFlow's ring buffer, which wraps after 30 minutes at 250 Hz.
• On Windows, connecting a Cyton now stops with an error unless the FTDI dongle's LatencyTimer is 1 ms. With the 16 ms default, samples reach the host in bursts, so BrainFlow's arrival timestamps, and the markers aligned to them, can be off by up to a batch of samples. OpenBCI recommends the same setting (https://docs.openbci.com/Troubleshooting/FTDI_Fix_Windows/), and doc/misc/ntcs_phase1_instructions.md already calls it "a major gotcha". Existing Windows users need to set it once per dongle, and again if a driver update resets it. The error message says where and links OpenBCI's guide, and the Cyton section of doc/getting_started/streaming.md now covers it.