getDeltaChannelHistory

Walk the parent chain returning per-channel writes + seed, used to reconstruct DeltaChannel state from checkpoint_writes.

For each requested channel, walks ancestors of the checkpoint identified by config (following parentConfig) and accumulates the pending writes for that channel. The walk terminates per-channel at the nearest ancestor whose channel_values[ch] is populated; that value is returned as seed. If the walk reaches the root without finding a stored value, seed is omitted from that channel's entry — the consumer treats the absence as "start empty".

Walks the parent chain (not list({ before })): for forked threads, only on-path ancestors contribute.

The default implementation walks getTuple + parentConfig once for all channels — each ancestor visited once, not once per channel. Savers with direct storage access (e.g. MemorySaver) override for performance; the return contract is fixed here.