v2.51.2

Changed

• Documentation only, no change to the application. docs/SPEC.md now records three calendar
  behaviours it did not carry: that choosing a view persists it while drilling into one does not
  (v2.51.0), how the view switcher and the calendar body relate as tablist and panel, and that the
  agenda shows today even when today is empty (both v2.51.1). The first of these is the reason the
  matching bug stayed invisible for a release: the specification described the intention, the code
  did more, and nobody reading the spec would have found a contradiction.