About: find the mail node's name with resolvePtr

dns.reverse came back empty inside the image while the resolver answered
the PTR, so About showed only the address. Ask for the PTR record of the
in-addr.arpa / ip6.arpa name directly.

server/src/nodes.test.ts

@@ -1,6 +1,6 @@
 import { test, beforeEach } from "node:test";
 import assert from "node:assert/strict";
-import { clearNodeCache, mailNode } from "./nodes.js";
+import { clearNodeCache, mailNode, ptrName } from "./nodes.js";
 
 /**
  * About names the mail node by the PTR of the address the server's name
@@ -38,3 +38,12 @@ test("the answer is cached for a minute, then looked up again", async () => {
   await mailNode("https://mail.example.com", count, rev, 61_000);
   assert.equal(calls, 2);
 });
+
+test("the PTR name is built for IPv4 and IPv6 alike", () => {
+  assert.equal(ptrName("192.0.2.52"), "52.2.0.192.in-addr.arpa");
+  assert.equal(
+    ptrName("2001:db8::25"),
+    "5.2.0.0.0.0.0.0.0.0.0.0.0.0.0.0.0.0.0.0.0.0.0.0.8.b.d.0.1.0.0.2.ip6.arpa",
+  );
+  assert.equal(ptrName("::1"), "1" + ".0".repeat(31) + ".ip6.arpa");
+});