feat(stats): add groupSessionsByVideo helper for episode rollups

Pure TS helper that buckets SessionSummary[] by videoId for the
Sessions tab collapsible UI (Task 8). Null videoIds become singleton
buckets keyed by sessionId. Covered by 4 node:test cases.

stats/src/lib/session-grouping.test.ts

@@ -0,0 +1,72 @@
+import assert from 'node:assert/strict';
+import test from 'node:test';
+import type { SessionSummary } from '../types/stats';
+import { groupSessionsByVideo } from './session-grouping';
+
+function makeSession(overrides: Partial<SessionSummary> & { sessionId: number }): SessionSummary {
+  return {
+    sessionId: overrides.sessionId,
+    canonicalTitle: null,
+    videoId: null,
+    animeId: null,
+    animeTitle: null,
+    startedAtMs: 1000,
+    endedAtMs: null,
+    totalWatchedMs: 0,
+    activeWatchedMs: 0,
+    linesSeen: 0,
+    tokensSeen: 0,
+    cardsMined: 0,
+    lookupCount: 0,
+    lookupHits: 0,
+    yomitanLookupCount: 0,
+    knownWordsSeen: 0,
+    knownWordRate: 0,
+    ...overrides,
+  };
+}
+
+test('empty input returns empty array', () => {
+  assert.deepEqual(groupSessionsByVideo([]), []);
+});
+
+test('two unique videoIds produce 2 singleton buckets', () => {
+  const sessions = [
+    makeSession({ sessionId: 1, videoId: 10, startedAtMs: 1000, activeWatchedMs: 100, cardsMined: 2 }),
+    makeSession({ sessionId: 2, videoId: 20, startedAtMs: 2000, activeWatchedMs: 200, cardsMined: 3 }),
+  ];
+  const buckets = groupSessionsByVideo(sessions);
+  assert.equal(buckets.length, 2);
+  const keys = buckets.map((b) => b.key).sort();
+  assert.deepEqual(keys, ['v-10', 'v-20']);
+  for (const bucket of buckets) {
+    assert.equal(bucket.sessions.length, 1);
+  }
+});
+
+test('two sessions sharing a videoId collapse into 1 bucket with summed totals and most-recent representative', () => {
+  const older = makeSession({ sessionId: 1, videoId: 42, startedAtMs: 1000, activeWatchedMs: 300, cardsMined: 5 });
+  const newer = makeSession({ sessionId: 2, videoId: 42, startedAtMs: 9000, activeWatchedMs: 500, cardsMined: 7 });
+  const buckets = groupSessionsByVideo([older, newer]);
+  assert.equal(buckets.length, 1);
+  const [bucket] = buckets;
+  assert.equal(bucket!.key, 'v-42');
+  assert.equal(bucket!.videoId, 42);
+  assert.equal(bucket!.sessions.length, 2);
+  assert.equal(bucket!.totalActiveMs, 800);
+  assert.equal(bucket!.totalCardsMined, 12);
+  assert.equal(bucket!.representativeSession.sessionId, 2); // most recent (highest startedAtMs)
+});
+
+test('sessions with null videoId become singleton buckets keyed by sessionId', () => {
+  const s1 = makeSession({ sessionId: 101, videoId: null, activeWatchedMs: 50, cardsMined: 1 });
+  const s2 = makeSession({ sessionId: 202, videoId: null, activeWatchedMs: 75, cardsMined: 2 });
+  const buckets = groupSessionsByVideo([s1, s2]);
+  assert.equal(buckets.length, 2);
+  const keys = buckets.map((b) => b.key).sort();
+  assert.deepEqual(keys, ['s-101', 's-202']);
+  for (const bucket of buckets) {
+    assert.equal(bucket.videoId, null);
+    assert.equal(bucket.sessions.length, 1);
+  }
+});