0301 — The waterline rollup stays lazy; no cursor update at commit time

Context. Commit today touches only lease and deliveries, never cursor; a lazy background poll (AdvanceWaterline) moves committed afterward, so retention drops and delivery sweeps trail real progress by up to the poll interval (a few seconds under the default ClaimPollRate). The alternative was updating cursor synchronously inside every Commit. The question had been deliberately deferred earlier and was resolved here by measurement.

Decision. Stay lazy: AdvanceWaterline remains a periodic background poll and Commit never touches cursor. Added WorkConsumerConfig.WaterlinePollRate (0 defaults to ClaimPollRate, same pattern as JanitorPollRate) so staleness is tunable without adding hot-path cost.

Consequences. Cursor-derived actions lag by up to WaterlinePollRate; an operator who needs less lag shortens the knob — a representative lab run measured catch-up time drop 15.6x (2.03s to 130ms) with a faster poll. Zero throughput cost preserved: concurrent committers in a group keep committing fully in parallel. Rejected: synchronous UPDATE cursor in Commit — it adds a row every concurrent committer in the group serializes on, measured 1.3x-1.9x slower at 20 concurrent committers; a permanent tax on every commit to buy a sub-5-second wait down to milliseconds, which retention drop decisions and a debug readout do not need.