gate: #712 (chtypes type layer) has merged #826

This issue stands for PR #712 being merged (the chtypes type layer). It closes automatically when #712 merges, because #712's description says it closes this issue.

The issues collected in the follow-up epic are marked blocked by this issue. Until it closes, they describe code that only exists on #712's branch, or behaviour that changes when #712 lands, so please don't start them yet. GitHub can't mark an issue as blocked by a pull request directly; this issue is that gate.