Per-channel counters since the last _DeltaSnapshot was written.
This metadata field backs DeltaChannel (beta). The key name and
contents may change while the delta-channel design stabilizes.
Maps channel name -> (updates, supersteps):
updates): number of supersteps that wrote to this channel
since its last snapshot blob.supersteps): total supersteps elapsed since this channel's
last snapshot, regardless of whether the channel was written.A snapshot fires when EITHER updates >= ch.snapshot_frequency OR
supersteps >= DELTA_MAX_SUPERSTEPS_SINCE_SNAPSHOT (system-wide bound,
default 5000, env LANGGRAPH_DELTA_MAX_SUPERSTEPS_SINCE_SNAPSHOT).
The supersteps bound prevents unbounded ancestor walks on threads where
a delta channel exists but is no longer being updated.
Absent on threads that don't use delta channels. Persisted as a 2-element list in JSON (no native tuple).