Sitemaps: every entry carries an honest lastmod (item changes, release time for static pages) (#42)

ddiff --git a/README.md b/README.md
iindex 40bfcd2..3ecf388 100644
---- a/README.md
++++ b/README.md
@@@ -24,6 +24,8 @@ citations, redirects, the indexability gate, feeds, sitemaps, JSON-LD, authorshi
  media references, discussion references and the Search index hooks all come from there. Wiki owns
  its publication state; the packages supply the mechanics.
  
++Sitemap entries carry lastmod from each published page or space's latest change.
++
  ## Owns
  
  The ten authority tables of §15.13, in Wiki's own PostgreSQL database (`ov_wiki` on the host's data role, ADR-035; schema in [migrations/](migrations/)):
ddiff --git a/server/http/machine.js b/server/http/machine.js
iindex afab749..57d45ca 100644
---- a/server/http/machine.js
++++ b/server/http/machine.js
@@@ -37,25 +37,39 @@ function createMachine({ svc, config }) {
  
      const wrap = (fn) => (req, res, next) => Promise.resolve(fn(req, res)).catch(next);
  
--    async function pageEntries() {
--        return (await svc.publishedPublic()).map(({ space, page, decision }) => ({
++    async function pageEntries(published = null) {
++        published = published || await svc.publishedPublic();
++        return published.map(({ space, page, decision }) => ({
              loc: svc.pageUrl(space, page), lastmod: page.revision_published_at, decision,
          }));
      }
  
--    async function spaceEntries() {
++    async function spaceEntries(published = null) {
++        published = published || await svc.publishedPublic();
++        const newestPage = new Map();
++        for (const { space, page } of published) {
++            if (!page.revision_published_at) continue;
++            newestPage.set(space.id, Math.max(newestPage.get(space.id) || 0, page.revision_published_at));
++        }
          return (await svc.listSpaces({ kind: 'anonymous' })).filter((s) => s.visibility === 'public').map((s) => {
              const url = seo.canonicalUrl(origin, svc.spacePath(s));
              // A space index is navigation, not an article: no word or source minimum.
              const decision = seo.evaluate({ state: 'published', visibility: 'public', canonicalUrl: url, wordCount: 0 }, { policy: { minWords: 0, requireSources: false }, now: Date.now() });
--            return { loc: url, lastmod: s.updated_at, decision };
++            return { loc: url, lastmod: Math.max(s.updated_at, newestPage.get(s.id) || 0), decision };
          });
      }
  
      router.get('/sitemap.xml', wrap(async (_req, res) => {
--        const pages = seo.sitemap(await pageEntries(), { maxUrls: PER_SITEMAP });
--        const maps = [{ loc: `${origin}/sitemaps/spaces.xml` }];
--        pages.files.forEach((_f, i) => maps.push({ loc: `${origin}/sitemaps/pages-${i + 1}.xml` }));
++        const published = await svc.publishedPublic();
++        const entries = (await pageEntries(published)).filter((e) => e.decision.indexable);
++        const spaces = (await spaceEntries(published)).filter((e) => e.decision.indexable);
++        const pages = seo.sitemap(entries, { maxUrls: PER_SITEMAP });
++        const newest = (rows) => {
++            const times = rows.map((e) => e.lastmod).filter(Boolean);
++            return times.length ? new Date(Math.max(...times)).toISOString() : undefined;
++        };
++        const maps = [{ loc: `${origin}/sitemaps/spaces.xml`, lastmod: newest(spaces) }];
++        pages.files.forEach((_f, i) => maps.push({ loc: `${origin}/sitemaps/pages-${i + 1}.xml`, lastmod: newest(entries.slice(i * PER_SITEMAP, (i + 1) * PER_SITEMAP)) }));
          setCache(res).type('application/xml').send(seo.sitemapIndex(maps));
      }));
      router.get('/sitemaps/spaces.xml', wrap(async (_req, res) => setCache(res).type('application/xml').send(seo.sitemap(await spaceEntries()).files[0])));
ddiff --git a/test/visibility.test.js b/test/visibility.test.js
iindex 26546f8..446ad2f 100644
---- a/test/visibility.test.js
++++ b/test/visibility.test.js
@@@ -6,6 +6,17 @@
   * search.index-document@1 (openvibe-contracts).
   */
  const assert = require('assert');
++
++function assertSitemapLastmods(xml, tag = 'url') {
++    const entries = [...xml.matchAll(new RegExp(`<${tag}>([\\s\\S]*?)</${tag}>`, 'g'))];
++    for (const [, body] of entries) {
++        const match = /<lastmod>(\d{4}-\d{2}-\d{2})<\/lastmod>/.exec(body);
++        assert.ok(match, body);
++        assert.strictEqual(new Date(match[1]).toISOString().slice(0, 10), match[1]);
++    }
++    return entries;
++}
++
  const contracts = require('openvibe-contracts');
  const H = require('./helpers');
  
@@@ -36,6 +47,11 @@ async function lastIndexEvent(h, pageId) {
          await h.svc.publish(unsourced.id, {}, owner);
          r = await H.req(h, 'GET', '/sitemaps/pages-1.xml');
          assert.ok(r.text.includes('http://wiki.test/w/notes/open-page'));
++        assertSitemapLastmods(r.text);
++        const publishedPage = await h.db.prepare('SELECT revision_published_at FROM wiki_pages WHERE id = ?').get(page.id);
++        assert.ok(r.text.includes(`<lastmod>${new Date(publishedPage.revision_published_at).toISOString().slice(0, 10)}</lastmod>`));
++        assertSitemapLastmods((await H.req(h, 'GET', '/sitemaps/spaces.xml')).text);
++        assertSitemapLastmods((await H.req(h, 'GET', '/sitemap.xml')).text, 'sitemap');
          assert.ok(!r.text.includes('/w/notes/unsourced-page'), 'an unsourced page is noindex: not in the sitemap');
          assert.ok((await H.req(h, 'GET', '/feed.atom')).text.includes('http://wiki.test/w/notes/open-page'));
          // /llms-full.txt: the llms.txt header, then exactly the indexable pages — the same gate as the