Follow-up from #19599 (comment): RNTuple snapshotting currently does not support fAutoFlush, which doesn't really map to RNTuple's target cluster size. We need to discuss how and if we want to support it.

Note that UntypedSnapshotTTreeHelperMT is also questionable in that regard because it writes / flushes output trees in FinalizeTask. So for RDataFrame snapshotting to TTree in MT mode, it's already more of an upper guarantee, but I guess many cluster will actually be smaller than the user wants them...