feat(exam9): link tasks to textbook + difficulty-ordered random + topic exclusion

Practice (random) now picks tasks by ascending difficulty so the first
slot is always level 1 and the session ramps up. Adds ?exclude= to drop
specific subtopics from the random pool, with a per-section checkbox
modal in the UI.

Each task carries a topic_ref (textbook chapter + paragraph) shown as
a 'Учить тему · §N' button next to the solution, deep-linking to the
right section of /textbook/<slug>. Mapping seeded for all 15 math9
subtopics in migration 028.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

frontend/js/exam-prep/task-card.js

@@ -34,6 +34,24 @@
     return String(s || '').replace(/[&<>"']/g, c => ({ '&':'&amp;', '<':'&lt;', '>':'&gt;', '"':'&quot;', "'":'&#39;' }[c]));
   }
 
+  /* topic_ref → "Учить тему" deep-link to the textbook chapter/paragraph.
+     ref = { slug, paragraph, title }. Paragraph is null for hub links. */
+  function buildRefLink(ref) {
+    if (!ref || !ref.slug) return '';
+    const href = ref.paragraph
+      ? `/textbook/${encodeURIComponent(ref.slug)}#sec-p${ref.paragraph}`
+      : `/textbook/${encodeURIComponent(ref.slug)}`;
+    const label = ref.paragraph ? `§${ref.paragraph}` : 'учебник';
+    const title = ref.title ? `Учить тему: ${escapeHtml(ref.title)}` : 'Перейти к материалу';
+    return `<a class="tc-ref-btn" href="${href}" target="_blank" rel="noopener" title="${title}">
+      <svg viewBox="0 0 24 24" fill="none" stroke="currentColor" stroke-width="2" stroke-linecap="round" stroke-linejoin="round" aria-hidden="true">
+        <path d="M2 3h6a4 4 0 0 1 4 4v14a3 3 0 0 0-3-3H2z"/>
+        <path d="M22 3h-6a4 4 0 0 0-4 4v14a3 3 0 0 1 3-3h7z"/>
+      </svg>
+      <span>Учить тему · ${escapeHtml(label)}</span>
+    </a>`;
+  }
+
   function buildOptsBlock(taskId, opts) {
     const isLong = opts.some(([, t]) => t.length > 40 && !t.startsWith('$'));
     const cls = isLong ? 'tc-opts tc-opts-vertical' : 'tc-opts';
@@ -101,11 +119,15 @@
         </div>` + verdictSlot;
     }
 
+    const refLink = buildRefLink(task.topic_ref);
     const solBlock = (showSol && task.solution) ? `
       <div class="tc-sol-wrap">
-        <button class="tc-sol-btn" data-tc-sol>${ICONS.chev}<span>Показать решение</span></button>
+        <div class="tc-sol-row">
+          <button class="tc-sol-btn" data-tc-sol>${ICONS.chev}<span>Показать решение</span></button>
+          ${refLink}
+        </div>
         <div class="tc-sol-panel" data-tc-sol-panel>${task.solution}</div>
-      </div>` : '';
+      </div>` : (refLink ? `<div class="tc-sol-wrap"><div class="tc-sol-row">${refLink}</div></div>` : '');
 
     card.innerHTML = `
       <div class="tc-head">