Stop borrowing Stalwart's version number

The middle field was the Stalwart generation a build targeted -- 16 for
0.16 -- which leaves nowhere to go when Stalwart reaches 1.0. There is no
honest value for it: 2.1 sorts below the 2.16 already deployed, so every
image and About screen would have read as a downgrade. Tying our
numbering to somebody else's was the mistake, and which Stalwart a build
needs is said properly in the README badge and KNOWN-ISSUES, where it can
be precise rather than one digit.

The version is now the date of the commit it was built from, and the pull
request moves after the + as build metadata. It is provenance rather than
a rank: at the rate they merge here it climbs without bound and says
nothing about how new a build is. Everything after the + is ignored when
versions are compared, which reads correctly -- two builds from the same
day differ in where they came from, not in age -- and nothing depends on
that comparison anyway, since images are pruned by creation time and a
rollback names a git ref.

The date is the commit's own, so rebuilding an old commit gives the
version it had the first time. package.json is no longer the source of
anything and sits at 0.0.0, which is what an unversioned build reports
and is meant to look wrong.

The formatting is a pure function now, so the rules have tests. They had
none while the version was the thing naming every image we ship.

scripts/version.d.mts

@@ -1,4 +1,5 @@
 /** Types for `version.mjs`, which is plain JS so the Dockerfile and shell can run it directly. */
-export function baseVersion(): string;
+export const UNVERSIONED: string;
+export function formatVersion(commit: { date: string; subject?: string; sha: string }): string;
 export function versionFromGit(): string | null;
 export function resolveVersion(): string;