A compact projection snapshot with the durable thread revision it covers.
Checkpoints are rebuild accelerators. The journal remains the source of truth,
and thread_rev records the last applied Jido thread revision so a projection
can resume from a precise point instead of silently depending on current code.
Summary
Types
@type t() :: %SquidMesh.Runtime.Journal.Checkpoint{ projection: term(), thread: SquidMesh.Runtime.DispatchProtocol.Entry.thread(), thread_id: String.t(), thread_rev: non_neg_integer(), updated_at: DateTime.t() }