deltaChannelsWithPendingWrites(
channels: Record<string, BaseChannel>,
pendingWrites: readonly [string, string, unknown][] DeltaChannels a branch starting from this checkpoint must snapshot.
A checkpoint's pending writes belong to the child that consumed them, and
nothing records which child that was. A new branch snapshots every delta
channel they touch, so its ancestor walk never replays them (port of
Python #8548's delta_channels_with_pending_writes).