sirius-x: release a name from the dashboard; the gateway reports a released name

The Transfer tab gains a Release card. It asks for the name to be typed,
refuses while the name is listed for sale, and says plainly what ends: the
certificate, the records, the ownership, with nothing refunded.

/api/name/<name> returns released_at for a name that left the index through
a release, so a client can tell 'never registered' from 'given back' and
whether the one-block gap has passed. The register flow uses the same signal
to say why a just-released name is not open yet instead of calling it taken.

Docs gain a Releasing a name section; the roadmap marks its first two steps
done.

src/gateway/public-gateway.mjs

@@ -1128,12 +1128,18 @@ const server = http.createServer(async (req, res) => {
       try {
         const key = normalizeName(raw); // "<label>.<tld>"
         const tld = key.split(".").pop();
-        const entry = (await getIndex()).get(key);
+        const index = await getIndex();
+        const entry = index.get(key);
+        // A released name (REL) is out of the index. `released_at` is the
+        // height of its release (<= 0 while unconfirmed); it can be
+        // registered again only from the block after that one.
+        const releasedAt = index.released?.get(key);
         res.writeHead(200);
         res.end(JSON.stringify({
           name: key, tld, registered: !!entry,
           records: entry?.records ?? null, category: entry?.category ?? null,
           owner: entry?.owner ?? null,
+          ...(releasedAt !== undefined ? { released_at: releasedAt, release_pending: !(releasedAt > 0) } : {}),
         }));
       } catch (e) {
         res.writeHead(400); res.end(JSON.stringify({ error: e.message }));