THALis: startup burn-in and DC removal

15 sec burn-in — wall time or actual samples received? reset behavior on reconnect too, don't carry an old running mean into a new stream

archived change: e752b94 adds burn-in state management to streaming, running-mean DC removal to preprocessing, and associated entry-point wiring. The recorded default burn-in is 15 seconds, with development runs 8–21 collected during this iteration.

reason for separating states: connecting a stream is different from declaring its samples usable for training or inference. The downstream pipeline needs to know whether data are within startup settling, accepted recording, or an interruption.

evidence boundary: the archive records the engineering change, not a quantified study demonstrating that 15 seconds is sufficient for every recording. Retain the value as a configuration default to inspect, not a universally established electrode-settling time.

archived code references: e752b94.

• review questions: is the 15-second interval measured in wall time or received samples? Are discarded samples still saved as raw data? Does the window buffer reset when burn-in finishes? How is the running-mean state initialized and updated after reconnecting?