v5.1.2

Build and release plumbing. Nothing changes in the app itself — if
you're on 5.1.1 there's no need to update.

Two checks turned out to cover less than they appeared to:

• The version-sync step updated every version-bearing file except the
  two Cargo.lock files, which also record a version for each of our
  own crates. Nothing failed loudly, so the symptom was that a freshly
  released tree went dirty the moment anyone built it. 5.1.1 shipped
  that way; this release carries the correction.
• Continuous integration checked formatting on the engine but never on
  the desktop app, so a handful of files there had drifted. Reformatted,
  and the check now covers both.