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/practice.js

@@ -15,6 +15,8 @@
   let batch        = null;     // { strategy, session_id, tasks: [...] }
   let strategy     = readStrategyFromUrl() || readPersistedStrategy() || 'random';
   let count        = readPersistedCount();
+  let excludeSlugs = readPersistedExclude();   // Set<slug>
+  let topicSections = null;    // { sections: [...] } — lazy-loaded
   let finalized    = false;
   const results    = new Map();   // taskId -> { isCorrect: 0|1|null, attempts: number }
 
@@ -39,6 +41,17 @@
   function persistCount(n) {
     try { localStorage.setItem(`exam_prep_${examKey}_practice_count`, String(n)); } catch {}
   }
+  function readPersistedExclude() {
+    try {
+      const raw = localStorage.getItem(`exam_prep_${examKey}_practice_exclude`) || '';
+      return new Set(raw.split(',').filter(Boolean));
+    } catch { return new Set(); }
+  }
+  function persistExclude(set) {
+    try {
+      localStorage.setItem(`exam_prep_${examKey}_practice_exclude`, Array.from(set).join(','));
+    } catch {}
+  }
 
   /* ── Boot: show controls and load first batch ───────────────── */
   await loadBatch();
@@ -47,6 +60,11 @@
   function controlsHTML() {
     const opts = [5, 10, 15, 20].map(n => `
       <option value="${n}" ${n === count ? 'selected' : ''}>${n} задач</option>`).join('');
+    const excludeBadge = excludeSlugs.size
+      ? `<span class="pr-exclude-count">${excludeSlugs.size}</span>` : '';
+    // Exclude only applies to the difficulty-ordered random pool. Hide
+    // for other strategies so users don't think it filters them too.
+    const showExclude = strategy === 'random';
     return `
       <div class="pr-controls">
         <div class="pr-controls-row">
@@ -66,10 +84,19 @@
             <label>Размер сессии:</label>
             <select class="pr-count-select">${opts}</select>
           </div>
+          ${showExclude ? `
+          <button class="ep-btn pr-exclude-btn" title="Исключить темы из пула случайных задач">
+            <i data-lucide="filter-x"></i> Исключить темы ${excludeBadge}
+          </button>` : ''}
           <button class="ep-btn ep-btn-primary pr-restart">
             <i data-lucide="rotate-cw"></i> Новая сессия
           </button>
         </div>
+        ${strategy === 'random' ? `
+          <div class="pr-difficulty-note">
+            <i data-lucide="bar-chart-2"></i>
+            Задачи идут по возрастанию сложности: 1-я всегда лёгкая, к концу — сложнее.
+          </div>` : ''}
         <div class="pr-progress" id="pr-progress"></div>
       </div>`;
   }
@@ -102,7 +129,11 @@
     wireControls();
 
     try {
-      batch = await EP.api.getPracticeNext(examKey, { count, strategy });
+      const query = { count, strategy };
+      if (strategy === 'random' && excludeSlugs.size) {
+        query.exclude = Array.from(excludeSlugs).join(',');
+      }
+      batch = await EP.api.getPracticeNext(examKey, query);
     } catch (e) {
       main.querySelector('.ep-empty').outerHTML = `
         <div class="ep-empty">
@@ -234,6 +265,77 @@
     }
     const restart = main.querySelector('.pr-restart');
     if (restart) restart.onclick = () => loadBatch();
+
+    const excludeBtn = main.querySelector('.pr-exclude-btn');
+    if (excludeBtn) excludeBtn.onclick = () => openExcludeModal();
+  }
+
+  /* ── Exclude-topics modal ───────────────────────────────────── */
+  async function openExcludeModal() {
+    if (!topicSections) {
+      try {
+        topicSections = await EP.api.listTopics(examKey);
+      } catch (e) {
+        LS.toast?.('Не удалось загрузить темы', 'error');
+        return;
+      }
+    }
+    const sections = topicSections.sections || [];
+    const sectionsHtml = sections.map(sec => {
+      const items = (sec.subtopics || []).map(st => {
+        const checked = excludeSlugs.has(st.slug) ? 'checked' : '';
+        return `
+          <label class="pr-ex-item">
+            <input type="checkbox" value="${st.slug}" ${checked} />
+            <span class="pr-ex-title">${escapeHtml(st.title)}</span>
+            <span class="pr-ex-meta">${st.total || 0} задач</span>
+          </label>`;
+      }).join('');
+      return `
+        <div class="pr-ex-section">
+          <div class="pr-ex-section-head">
+            <span>${escapeHtml(sec.title)}</span>
+            <button type="button" class="pr-ex-toggle-all" data-section="${sec.slug}">Все</button>
+          </div>
+          <div class="pr-ex-items" data-section-items="${sec.slug}">${items}</div>
+        </div>`;
+    }).join('');
+
+    const body = `
+      <div class="pr-ex-form">
+        <p class="pr-ex-hint">Отмеченные темы не попадут в случайный пул. Сложность распределения сохраняется.</p>
+        ${sectionsHtml || '<div class="pr-ex-empty">Темы не настроены</div>'}
+      </div>`;
+
+    const m = LS.modal({
+      title: 'Исключить темы из пула',
+      content: body,
+      size: 'md',
+      actions: [
+        { label: 'Сбросить', onClick: () => {
+            m.body.querySelectorAll('input[type="checkbox"]').forEach(c => c.checked = false);
+          } },
+        { label: 'Отмена', onClick: () => m.close() },
+        { label: 'Применить', primary: true, onClick: () => {
+            const checked = Array.from(m.body.querySelectorAll('input[type="checkbox"]:checked')).map(c => c.value);
+            excludeSlugs = new Set(checked);
+            persistExclude(excludeSlugs);
+            m.close();
+            loadBatch();
+          } },
+      ],
+    });
+
+    // "Все" toggle per section
+    m.body.querySelectorAll('.pr-ex-toggle-all').forEach(btn => {
+      btn.onclick = () => {
+        const items = m.body.querySelector(`[data-section-items="${btn.dataset.section}"]`);
+        if (!items) return;
+        const boxes = items.querySelectorAll('input[type="checkbox"]');
+        const allChecked = Array.from(boxes).every(b => b.checked);
+        boxes.forEach(b => b.checked = !allChecked);
+      };
+    });
   }
 
   function escapeHtml(s) {