Persists a stage tally to scan_runs at a bounded cadence — every
flushEvery items OR every flushPeriod elapsed, whichever fires
first. Caller supplies marshal + persist closures so each stage owns
its own tally type. Tick swallows mid-stage persist errors silently
(observability bug, not scan failure); Flush propagates errors so
the orchestrator's captureErr can surface them.
Tests cover: count-based flush, time-based flush, Flush always
persists, Tick swallows errors and keeps firing, Flush propagates
errors, counters reset after each flush.
bvandeusen
7b9ea131a2