0166 — Waterline advance moved to a lazy roller; per-message MoveCursor deleted

Context. The first cursor-path implementation advanced committed per message on the hot path via MoveCursor. With ranges owned by leases, committed can no longer be advanced message-by-message anyway — it must respect open leases.

Decision. All waterline motion moved to RollWaterline, a dedicated goroutine that ticks on PollRate and calls AdvanceWaterline off the hot path. CursorClaim claims a ClaimedRange, processes the whole range, and calls CommitRange(token), which only frees the lease; the hot path never touches committed. MoveCursor was deleted.

Consequences. committed lags in-flight reality by up to one roller tick — acceptable because it is a durability frontier, not a scheduling input. Success costs zero writes to the cursor row per message; the cursor row is contended only by the roller and the claim’s claimed advance.