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/css/exam-prep.css

@@ -1054,3 +1054,84 @@
   .tc-input-row { gap: 8px; }
   .tc-ans-input { max-width: 100%; }
 }
+
+/* ── "Учить тему" link inside a task card ───────────────────────── */
+.tc-sol-row {
+  display: flex; align-items: center; gap: 10px;
+  flex-wrap: wrap;
+}
+.tc-ref-btn {
+  display: inline-flex; align-items: center; gap: 6px;
+  margin-left: auto;
+  padding: 6px 12px;
+  border: 1.5px solid var(--violet, #7c3aed);
+  border-radius: 9px;
+  background: rgba(124, 58, 237, 0.08);
+  color: var(--violet, #7c3aed);
+  font-family: 'Manrope', sans-serif; font-size: .82rem; font-weight: 700;
+  text-decoration: none;
+  transition: background .15s, color .15s;
+}
+.tc-ref-btn:hover {
+  background: var(--violet, #7c3aed); color: #fff;
+}
+.tc-ref-btn svg { width: 14px; height: 14px; }
+
+/* ── Difficulty-progression note above the task list ─────────────── */
+.pr-difficulty-note {
+  display: flex; align-items: center; gap: 8px;
+  margin-top: 10px;
+  padding: 8px 12px;
+  background: rgba(6, 214, 160, 0.08);
+  border-left: 3px solid #06D6A0;
+  border-radius: 6px;
+  font-size: .82rem;
+  color: var(--text-2);
+}
+.pr-difficulty-note i, .pr-difficulty-note svg { width: 14px; height: 14px; flex-shrink: 0; }
+
+/* ── Exclude-topics button + count badge ─────────────────────────── */
+.pr-exclude-btn {
+  display: inline-flex; align-items: center; gap: 6px;
+  position: relative;
+}
+.pr-exclude-count {
+  display: inline-flex; align-items: center; justify-content: center;
+  min-width: 18px; height: 18px;
+  padding: 0 5px;
+  border-radius: 9px;
+  background: var(--violet, #7c3aed); color: #fff;
+  font-size: .7rem; font-weight: 800; line-height: 1;
+}
+
+/* ── Exclude-topics modal ────────────────────────────────────────── */
+.pr-ex-form { display: flex; flex-direction: column; gap: 16px; max-height: 60vh; overflow-y: auto; }
+.pr-ex-hint { font-size: .82rem; color: var(--text-3); margin: 0; }
+.pr-ex-section { border: 1px solid var(--border); border-radius: 10px; padding: 10px 12px; }
+.pr-ex-section-head {
+  display: flex; align-items: center; justify-content: space-between;
+  margin-bottom: 8px;
+  font-family: 'Unbounded', sans-serif; font-size: .82rem; font-weight: 800;
+  letter-spacing: .02em; color: var(--text);
+}
+.pr-ex-toggle-all {
+  border: 1px solid var(--border-h); background: transparent; color: var(--text-2);
+  padding: 3px 10px; border-radius: 6px;
+  font-family: 'Manrope', sans-serif; font-size: .72rem; font-weight: 700;
+  cursor: pointer; transition: all .12s;
+}
+.pr-ex-toggle-all:hover { border-color: var(--violet); color: var(--violet); }
+.pr-ex-items { display: flex; flex-direction: column; gap: 4px; }
+.pr-ex-item {
+  display: flex; align-items: center; gap: 10px;
+  padding: 6px 8px;
+  border-radius: 6px;
+  cursor: pointer;
+  font-size: .9rem;
+  transition: background .12s;
+}
+.pr-ex-item:hover { background: var(--border); }
+.pr-ex-item input { accent-color: var(--violet); flex-shrink: 0; }
+.pr-ex-title { flex: 1; }
+.pr-ex-meta { font-size: .76rem; color: var(--text-3); }
+.pr-ex-empty { padding: 16px; text-align: center; color: var(--text-3); font-size: .85rem; }

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) {

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">