feat(phase3): learning helpers — summary, glossary, flashcards (SM-2), quizzes

Deterministic, on-device, no model:
- src/lib/learn pure modules (tokenize, summary [TextRank-ish], glossary
  [definition-pattern + frequency], flashcards [cloze/Q-A], srs [SM-2], quiz
  [MCQ with distractors]) — 37 unit tests.
- Flashcard persistence: Dexie v4 + native v4 `flashcards` table; create/list/
  listDue/updateSrs/delete/counts; cascades (transcript delete, course->Unsorted).
- UI: transcript "Study aids" (generate summary+glossary, click-to-seek; create
  flashcards), Study screen (SM-2 review + Anki CSV export), per-lecture Quiz,
  library Study link with due-count badge.

215 tests green, 0 tsc errors, web export builds.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

src/lib/learn/srs.test.ts

@@ -0,0 +1,82 @@
+import { describe, it, expect } from 'vitest';
+import { initialSrs, review } from './srs';
+
+const MS_PER_DAY = 86_400_000;
+const NOW = 1_700_000_000_000;
+
+describe('initialSrs', () => {
+  it('creates a neutral, immediately-due card', () => {
+    const s = initialSrs(NOW);
+    expect(s).toEqual({
+      ease: 2.5,
+      intervalDays: 0,
+      reps: 0,
+      lapses: 0,
+      due: NOW,
+    });
+  });
+});
+
+describe('review', () => {
+  it('does not mutate the input state', () => {
+    const s = initialSrs(NOW);
+    const frozen = { ...s };
+    review(s, 2, NOW);
+    expect(s).toEqual(frozen);
+  });
+
+  it('produces monotonically increasing intervals on repeated "good"', () => {
+    let s = initialSrs(NOW);
+    const intervals: number[] = [];
+    let t = NOW;
+    for (let i = 0; i < 5; i++) {
+      s = review(s, 2, t);
+      intervals.push(s.intervalDays);
+      t += s.intervalDays * MS_PER_DAY;
+    }
+    // 1 day, 6 days, then *ease each time -> strictly increasing.
+    for (let i = 1; i < intervals.length; i++) {
+      expect(intervals[i]!).toBeGreaterThan(intervals[i - 1]!);
+    }
+    expect(intervals[0]).toBe(1);
+    expect(intervals[1]).toBe(6);
+  });
+
+  it('increments reps on passing grades and resets on "again"', () => {
+    let s = initialSrs(NOW);
+    s = review(s, 2, NOW); // good
+    s = review(s, 2, NOW); // good
+    expect(s.reps).toBe(2);
+    s = review(s, 0, NOW); // again
+    expect(s.reps).toBe(0);
+    expect(s.lapses).toBe(1);
+  });
+
+  it('floors ease at 1.3 even after many "again" reviews', () => {
+    let s = initialSrs(NOW);
+    for (let i = 0; i < 20; i++) s = review(s, 0, NOW);
+    expect(s.ease).toBe(1.3);
+  });
+
+  it('lowers ease on hard, raises it on easy', () => {
+    const good = review(initialSrs(NOW), 2, NOW);
+    expect(good.ease).toBe(2.5);
+    const hard = review(initialSrs(NOW), 1, NOW);
+    expect(hard.ease).toBeCloseTo(2.35, 5);
+    const easy = review(initialSrs(NOW), 3, NOW);
+    expect(easy.ease).toBeCloseTo(2.65, 5);
+  });
+
+  it('computes due = now + intervalDays * 86400000 and sets lastReviewed', () => {
+    const s = review(initialSrs(NOW), 2, NOW);
+    expect(s.intervalDays).toBe(1);
+    expect(s.due).toBe(NOW + 1 * MS_PER_DAY);
+    expect(s.lastReviewed).toBe(NOW);
+  });
+
+  it('gives "easy" a longer interval than "good" at the same step', () => {
+    const good = review(initialSrs(NOW), 2, NOW);
+    const easy = review(initialSrs(NOW), 3, NOW);
+    expect(easy.intervalDays).toBeGreaterThan(good.intervalDays);
+  });
+});