inspect: route RunMulti start failures through terminal markers

Queue Start-failure terminal markers and let the dispatch loop write startErr and finishedAt, matching the terminal-marker protocol used for Wait results. The setup loop still initializes immutable state only; all terminal state now flows through fanIn. Start-error markers are sent from the closer goroutine so setup cannot block if an early agent fills fanIn before dispatch starts.

Adds coverage for the all-start-errors case to prove RunMulti still finishes and reports each start failure.