Interface PurgeExpiredThreadStateOptions

Clock override for tests.

Must match createD1Storage's tablePrefix.

Thread-state rows (state-signal lanes + goals) whose updatedAt is older than this expire. An actively-updated goal or task lane bumps updatedAt on every write, so it never ages out; an abandoned thread's state does. No default (opt-in, like the notification and thread TTLs — durable state is kept until the host sets a window).