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## Summary Add a user-facing design doc and `get_delta_channel_keepset` helper for third-party `BaseCheckpointSaver` authors who need to support graphs using `DeltaChannel`. **Deliverables:** 1. ~~**`docs/delta-channel-checkpointer-guide.md`** — comprehensive guide covering~~: moved to docs repo 2. **`BaseCheckpointSaver.get_delta_channel_keepset` / `aget_delta_channel_keepset`** — returns the minimum set of ancestor `checkpoint_id`s that must survive deletion for a given head's `DeltaChannel` reconstruction to remain intact. Enables safe `prune` implementations without silently corrupting delta history. 3. **Docstring warnings** on `prune`, `aprune`, `delete_for_runs`, `adelete_for_runs`, `copy_thread`, `acopy_thread` explaining the DeltaChannel pitfall (silent data loss if ancestor writes/snapshots are deleted). 4. **Three new conformance capabilities** in `libs/checkpoint-conformance`: - `delta_channel_history` — validates the `aget_delta_channel_history` walk contract - `delta_channel_keepset` — validates the keep-set contract - `delta_channel_reconstruction` — end-to-end round-trip (aput + aput_writes + history + reconstruct) ## Test plan - [x] `make format lint` passes in `libs/checkpoint`, `libs/checkpoint-conformance` - [x] All three new conformance capabilities pass against `InMemorySaver` - [x] Run conformance against SQLite saver - [x] Run conformance against Postgres saver --------- Co-authored-by: Cursor <cursoragent@cursor.com>