checksum yielding may be insufficient

In https://github.com/block/spirit/issues/426 we proposed (and implemented) a 24hr yield so that there is at least an upper bound on transaction age, even though it is 24hrs by default.. which is very long.

We are not about to revert this, but we discussed internally we should also monitor the HLL delta from when the checksum started, and have some mechanism where it decides to yield based on some threshold.

It's a feature request, but also a bug, since HLL issues can cause big problems in some cases. Looking for a prescriptive way to implement it, since HLL length is not a perfect metric.

The issue names no files, tests, or entry points. Start by tracing the existing 24-hour checksum yield described in issue #426 and the HLL delta monitoring discussed here; agree on a threshold and completion criteria with maintainers, since the metric and implementation approach remain unspecified.

Written by the indexing model from the issue text.