Checks the adapter capabilities and prefix without creating tables.
The anchor row is locked first, so an append and a fence of one run never interleave: a fence that commits first is seen, a fence that waits sees the turn and purges it.
Allocates the next run sequence and stores the turn atomically, rejecting fenced runs.
Reads turns after the cursor in sequence order.
Durable transcript turns, allocated and written under the same anchor as the content fence. A turn's start and end are kept in whole milliseconds, rounded.