resultSink

when supplied, receives each problem's result as it completes rather than only at the end, so a long study is checkpointed per problem instead of holding everything in heap and writing once. A sink also enables resume: a re-run attaches to an unfinished record of the same name and skips the problems already in it. run() still returns a BenchmarkSummary, so existing callers are unaffected — but note that on a resumed run the summary covers only the problems THIS pass ran; the skipped ones are in the sink, not in the returned value.