Add Blog RSS feed, footer feed/sitemap links, and expanded docs

Blog RSS feed (novaconium/ISSUES.md):
- App/pages/blog/feed/index.php - main feed, built from the same
  hand-written $posts array App/pages/blog/index.php renders from, so it
  works with content_index_enabled left at its default false. Added a
  published date field per post entry.
- App/pages/blog/tag/[tag]/feed/index.php - per-tag feed, gated on
  content_index_enabled the same way blog/tag/[tag]/index.php is.
- novaconium/lib/Rss.php - shared RSS 2.0 envelope builder both feeds
  use, generic (title/link/description/items in, XML string out).
- New head_extra block in the root layout (empty by default) so
  App/pages/blog/_layout/layout.twig can add feed auto-discovery scoped
  to /blog/* only.

Footer:
- New content_index_enabled Twig global (Renderer/bootstrap.php), same
  pattern as admin_auth_enabled.
- Right-aligned footer menu linking to /sitemap.xml (only shown when
  content_index_enabled, so it never links to a 404) and /blog/feed
  (always shown, no content-index dependency).

Docs:
- New /admin/docs/rss page: Lib\Rss API, the two shipped feeds, a worked
  example of adding a feed for another content collection, and how to
  advertise multiple feeds via head_extra.
- New /admin/docs/sitemap page: changefreq/priority blocks, what's
  included/excluded, an honest callout on the relative-<loc>-URL spec
  deviation (consistent with how canonical/og:url already work) with an
  override path for strict compliance, and submitting it to search
  engines.
- /admin/docs (Overview) gains a Requirements section: minimum
  requirements, then optional requirements for the database/content-index
  features (pdo_sqlite, optional pdo_mysql, FTS5) - and a Documentation
  heading separating it from the doc links list.
- /admin/docs/getting-started's Requirements line was stale (missing
  pdo_sqlite entirely, unlike README) - fixed and cross-linked to the
  Overview page's fuller list.
- /admin/docs/content-index trimmed to point at the new RSS/sitemap pages
  instead of duplicating their detail.

Also adds an "In-house comments" entry to novaconium/ISSUES.md's Backlog
- a Lib\ class for sidecar-attached comments on any page, depending on
  the not-yet-built Admin login & user management for real user accounts.

Closes the "Blog RSS feed" backlog item.

novaconium/lib/Rss.php

@@ -0,0 +1,49 @@
+<?php
+
+namespace Lib;
+
+/**
+ * A minimal RSS 2.0 envelope builder — plain string concatenation, no
+ * DOMDocument, same style as novaconium/pages/sitemap.xml/index.php.
+ * Generic on purpose (title/link/description/items in, XML string out) —
+ * it doesn't know about blog posts specifically; App/pages/blog/feed/ and
+ * App/pages/blog/tag/[tag]/feed/ are the two call sites that supply
+ * blog-shaped data to it.
+ *
+ * Links/guids are expected to be site-relative paths (e.g.
+ * "/blog/hello-world"), consistent with how this framework already
+ * handles canonical/og:url (see /admin/docs/seo) — there's no site-wide
+ * base-URL config to build absolute URLs from. Every <guid> is emitted
+ * with isPermaLink="false" for exactly this reason: it's a stable
+ * identifier, not a real absolute permalink.
+ */
+final class Rss
+{
+    /**
+     * @param array<int, array{title: string, link: string, guid: string, pubDateTimestamp: int, description: string}> $items
+     */
+    public static function render(string $channelTitle, string $channelLink, string $channelDescription, array $items): string
+    {
+        $xml = '<?xml version="1.0" encoding="UTF-8"?>' . "\n";
+        $xml .= '<rss version="2.0">' . "\n";
+        $xml .= '  <channel>' . "\n";
+        $xml .= '    <title>' . htmlspecialchars($channelTitle, ENT_XML1) . '</title>' . "\n";
+        $xml .= '    <link>' . htmlspecialchars($channelLink, ENT_XML1) . '</link>' . "\n";
+        $xml .= '    <description>' . htmlspecialchars($channelDescription, ENT_XML1) . '</description>' . "\n";
+
+        foreach ($items as $item) {
+            $xml .= '    <item>' . "\n";
+            $xml .= '      <title>' . htmlspecialchars($item['title'], ENT_XML1) . '</title>' . "\n";
+            $xml .= '      <link>' . htmlspecialchars($item['link'], ENT_XML1) . '</link>' . "\n";
+            $xml .= '      <guid isPermaLink="false">' . htmlspecialchars($item['guid'], ENT_XML1) . '</guid>' . "\n";
+            $xml .= '      <pubDate>' . date(DATE_RSS, $item['pubDateTimestamp']) . '</pubDate>' . "\n";
+            $xml .= '      <description>' . htmlspecialchars($item['description'], ENT_XML1) . '</description>' . "\n";
+            $xml .= '    </item>' . "\n";
+        }
+
+        $xml .= '  </channel>' . "\n";
+        $xml .= '</rss>' . "\n";
+
+        return $xml;
+    }
+}