Checkpoint

One durable point in a thread's execution - data only. Closures, codecs and update functions are rebound from the compiled graph on restore; every value inside carries the version of the codec that wrote it (VersionedJson).

parent is the checkpoint this one supersedes. A checkpointer accepts a new checkpoint only if parent is the thread's latest, so two writers cannot both advance one thread: the latest checkpoint's id is the thread's version. fencingToken is the token of the claim whose run wrote it (RunClaim); None on checkpoints written before claims existed (format 4 and earlier).

Persist with Checkpoint.toJson and read with Checkpoint.fromJson, which migrates older formatVersions and refuses newer ones.