fix(textbook): рабочий deep-link к § (/textbook/<slug>#sec-pN открывает нужный §)

Раньше статические страницы алгебры/геометрии игнорировали location.hash (init всегда
goTo('p10')), а textbook-tracker матчил только #pN через .para-pill — поэтому ссылки
exam-prep вида #sec-pN вели на главу, но не на §.

- server.js: /textbook/:slug всегда инжектит хелпер (и в обычном режиме, и в embed),
  _renderEmbed → _renderTextbook (кэш по filePath|mode, заголовки no-store сохранены).
- frontend/js/textbook-deeplink.js: по #sec-pN / #pN кликает .psel-card[data-id]
  (фолбэк .para-pill[data-para] → goTo → scrollIntoView). Универсально для статических
  и движковых страниц, идемпотентно, не конфликтует с textbook-tracker.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

backend/src/server.js

@@ -421,16 +421,22 @@ const EMBED_INJECT = `
 </script>
 `;
 
-function _renderEmbed(filePath, slug) {
+// Always injected (plain + embed): deep-link helper so /textbook/<slug>#sec-pN
+// actually opens § N. Without it the page ignores the hash and shows §1.
+const DEEPLINK_INJECT = `\n<script defer src="/js/textbook-deeplink.js"></script>\n`;
+
+function _renderTextbook(filePath, slug, embed) {
   let stat; try { stat = fs.statSync(filePath); } catch { return null; }
-  const cached = _embedCache.get(filePath);
+  const cacheKey = `${filePath}|${embed ? 'e' : 'p'}`;
+  const cached = _embedCache.get(cacheKey);
   if (cached && cached.mtime === stat.mtimeMs && cached.slug === slug) return cached.html;
   let html;
   try { html = fs.readFileSync(filePath, 'utf8'); } catch { return null; }
-  const inject = EMBED_INJECT.replace('__LS_SLUG__', JSON.stringify(slug));
+  let inject = DEEPLINK_INJECT;
+  if (embed) inject += EMBED_INJECT.replace('__LS_SLUG__', JSON.stringify(slug));
   if (html.includes('</head>')) html = html.replace('</head>', inject + '</head>');
   else html = inject + html;
-  _embedCache.set(filePath, { mtime: stat.mtimeMs, slug, html });
+  _embedCache.set(cacheKey, { mtime: stat.mtimeMs, slug, html });
   return html;
 }
 
@@ -443,13 +449,10 @@ app.get('/textbook/:slug', (req, res, next) => {
   res.setHeader('Cache-Control', 'no-store, no-cache, must-revalidate');
   res.setHeader('Pragma', 'no-cache');
   res.setHeader('Expires', '0');
-  if (req.query.embed === '1') {
-    const html = _renderEmbed(filePath, req.params.slug);
-    if (html == null) return next();
-    res.setHeader('Content-Type', 'text/html; charset=utf-8');
-    return res.send(html);
-  }
-  res.sendFile(filePath, err => { if (err) next(); });
+  const html = _renderTextbook(filePath, req.params.slug, req.query.embed === '1');
+  if (html == null) return next();
+  res.setHeader('Content-Type', 'text/html; charset=utf-8');
+  return res.send(html);
 });
 
 // Catalog: /textbooks → frontend/textbooks.html (explicit to avoid conflict with /textbooks/ directory)