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.

server/src/version.test.ts

@@ -0,0 +1,63 @@
+import { test } from "node:test";
+import assert from "node:assert/strict";
+import { formatVersion, resolveVersion, UNVERSIONED, versionFromGit } from "../../scripts/version.mjs";
+
+/**
+ * The version is this build's public identity: it names the image, and it is
+ * what About and /api/health report. It had no tests while it was
+ * `2.16.<pr>`; it has them now that the rules moved.
+ */
+
+test("a pull request merge is named by its number", () => {
+  assert.equal(
+    formatVersion({ date: "2026-08-30", subject: "Merge pull request #129 from LINUXexpert-org/link-project-site-v2", sha: "1fa6578" }),
+    "2026.8.30+pr129",
+  );
+});
+
+test("a commit that did not come through a pull request carries its SHA", () => {
+  // Claiming the last PR would say it *is* that PR rather than something after it.
+  assert.equal(formatVersion({ date: "2026-08-30", subject: "Fix a thing directly on main", sha: "1fa6578" }), "2026.8.30+g1fa6578");
+});
+
+test("leading zeros are stripped, since a version field may not carry them", () => {
+  assert.equal(formatVersion({ date: "2026-09-05", subject: "Merge pull request #7 from x/y", sha: "abc1234" }), "2026.9.5+pr7");
+  assert.equal(formatVersion({ date: "2027-01-01", subject: "", sha: "abc1234" }), "2027.1.1+gabc1234");
+});
+
+test("it sorts forward from the versions it replaces", () => {
+  // 2.16.129 was deployed. 2.1.x would have read as a downgrade, which is the
+  // whole reason the Stalwart generation left the version.
+  const [older, newer] = ["2.16.129", "2026.8.30"].map((v) => v.split(".").map(Number));
+  assert.ok(newer![0]! > older![0]!, "the leading field has to increase");
+});
+
+test("two builds from the same day differ, even though they rank the same", () => {
+  const a = formatVersion({ date: "2026-08-30", subject: "Merge pull request #128 from x/y", sha: "aaaaaaa" });
+  const b = formatVersion({ date: "2026-08-30", subject: "Merge pull request #129 from x/y", sha: "bbbbbbb" });
+  assert.notEqual(a, b);
+  assert.equal(a.split("+")[0], b.split("+")[0]);
+});
+
+test("the same commit always resolves to the same version", () => {
+  // Built from the commit's own date, not today's, so an old commit rebuilt
+  // now reports what it reported then.
+  const commit = { date: "2026-08-30", subject: "Merge pull request #129 from x/y", sha: "1fa6578" };
+  assert.equal(formatVersion(commit), formatVersion(commit));
+});
+
+test("an explicit IHASMAIL_VERSION wins, because the Docker build has no git", () => {
+  const before = process.env.IHASMAIL_VERSION;
+  process.env.IHASMAIL_VERSION = "2026.8.30+pr129";
+  try {
+    assert.equal(resolveVersion(), "2026.8.30+pr129");
+  } finally {
+    if (before === undefined) delete process.env.IHASMAIL_VERSION;
+    else process.env.IHASMAIL_VERSION = before;
+  }
+});
+
+test("a checkout with git resolves to a real version, and an unversioned build looks wrong", () => {
+  assert.match(versionFromGit() ?? "", /^\d{4}\.\d{1,2}\.\d{1,2}\+(pr\d+|g[0-9a-f]+)$/);
+  assert.equal(UNVERSIONED, "0.0.0");
+});