inspect: reserve fan-in capacity for start-failure terminals

Keep the start-failure terminal-marker protocol but reserve channel capacity for all possible Start-failure terminal markers in addition to event-burst slack. The fanIn buffer is now 17N: 16N for event jitter plus N terminal slots, so queued start terminals cannot consume the event-burst budget before dispatch starts.

Add a regression with a noisy started agent plus many Start failures to prove RunMulti drains and finishes.