fix(dispatch): window reachable checkpoints by in-range commit time

Bugbot review: reachableCheckpointIDsInRange recorded each checkpoint's latest referencing commit time with no upper bound, then the caller dropped entries outside [since, until). A checkpoint referenced by both an in-window commit and a later out-of-window commit stored the later time and was wrongly omitted despite qualifying in-window activity (reachable with an explicit past --until).

Bound the scan with --until and, authoritatively, skip any commit outside [since, until) in the parse loop, recording the max in-window time. A checkpoint is now present iff it has at least one in-window referencing commit, with an in-window timestamp.