FlowDrop 2.4.1

A rule-conformance patch. The specification registry's rules were read
against the code that claims to implement them, and every disagreement was
closed: sixteen fixes, no new features, no @api changes.

No database update and no config re-export. Seven behaviour changes are
documented in UPGRADE.md (2.4.x to 2.4.1) — read them before deploying, since
a rule the code was breaking is a rule a graph may have been leaning on.