Support an `enum EnforceThrottles { YES, NO }` dispatch mode that defaults to `NO`.

Update `HandleContextImpl.dispatchSyntheticTxn()` to:
- [ ] Accept a new `enum EnforceThrottles { YES, NO }` that defaults to `NO`.
- [ ] Create a `UsageSnapshotStack` analogous to the `RecordListBuilder`; instead of appending `RecordBuilder`'s as dispatches occur, take usage snapshots at the beginnings of dispatches, and support reverting to a particular snapshot.
- [ ] When a child dispatch is throttled, set the child record builder status to `BUSY`.

Start by reading HandleContextImpl.dispatchSyntheticTxn() and the existing RecordListBuilder behavior. Trace how dispatches and RecordBuilder statuses are handled, then define the UsageSnapshotStack flow so snapshots can be taken at dispatch starts and reverted; done means the enum defaults to NO and throttled child dispatches receive BUSY status.

Written by the indexing model from the issue text.