Mark Read

Mark the thread read through seq. A request, not a value: the server keeps the watermark monotonic and clamps it to a row that exists, so re-sending an old visit's seq, or a seq from the future, is safe.

GET /harness/unread and POST /harness/read's answer: how many coach-side rows (assistant / error / checkin_ask — never user or routine) sit past the pair's read watermark, and the watermark.