deltaChannelsToSnapshot

Return the set of DeltaChannel names that should snapshot now.

A channel snapshots when EITHER its accumulated update count reaches snapshotFrequency OR the total supersteps since its last snapshot reaches DELTA_MAX_SUPERSTEPS_SINCE_SNAPSHOT. Given channelVersions, a channel without a version is skipped: it was never written on this branch, so it has nothing to snapshot. Pass undefined on a branch without DELTA_WRITES_VERSIONED, where such a channel can still hold writes and this snapshot is what bounds its walk. Pure predicate, no mutation.