chore: Halkyon Learn moves into apps/learn

Its server, client, app and curator, and the tools that write its
assets (maps, Anki fixtures, icons), as they are; paths are fixed in
the commits that follow.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
Signed-off-by: Tizian 「ティツィアーン」 <raisondetredev@pm.me>

apps/learn/halkyon_learn_flutter/test/feature/statistics/model/study_statistics_test.dart

@@ -0,0 +1,108 @@
+import 'package:flutter_test/flutter_test.dart';
+import 'package:halkyon_learn_flutter/src/feature/statistics/model/study_statistics.dart';
+import 'package:halkyon_learn_flutter/src/feature/study/model/rating.dart';
+import 'package:halkyon_learn_flutter/src/feature/study/model/review.dart';
+
+void main() {
+  // Monday, 12 October 2026, in the afternoon, local time.
+  final now = DateTime(2026, 10, 12, 15);
+  var ids = 0;
+
+  Review review(
+    DateTime at, {
+    Duration duration = const Duration(seconds: 20),
+    Rating rating = Rating.good,
+    StudyMode mode = StudyMode.review,
+  }) => Review(
+    id: 'review-${ids++}',
+    cardId: 'card',
+    rating: rating,
+    reviewedAt: at.toUtc(),
+    duration: duration,
+    mode: mode,
+  );
+
+  test('counts at most a minute a card, cramming too, a fresh start '
+      'not', () {
+    final statistics = studyStatistics(
+      [
+        review(now, duration: const Duration(minutes: 45)),
+        review(now, duration: const Duration(seconds: 30)),
+        review(now, mode: StudyMode.cram),
+        review(
+          now,
+          duration: const Duration(minutes: 5),
+          mode: StudyMode.reset,
+        ),
+      ],
+      now: now,
+      span: StatisticsSpan.days,
+    );
+
+    expect(statistics.total, const Duration(seconds: 110));
+    expect(statistics.periods.last.time, const Duration(seconds: 110));
+    expect(statistics.periods.last.reviews, 3);
+    expect(statistics.studied, true);
+  });
+
+  test('per day counts every day from the first studied to today; the '
+      'record is the most in one day', () {
+    final statistics = studyStatistics(
+      [
+        // Three days ago, 2 minutes; today, 1.
+        for (var i = 0; i < 2; i++)
+          review(
+            now.subtract(const Duration(days: 3)),
+            duration: const Duration(minutes: 1),
+          ),
+        review(now, duration: const Duration(minutes: 1)),
+      ],
+      now: now,
+      span: StatisticsSpan.days,
+    );
+
+    expect(statistics.total, const Duration(minutes: 3));
+    // Four days: three ago, two, one, today.
+    expect(statistics.perDay, const Duration(seconds: 45));
+    expect(statistics.record, const Duration(minutes: 2));
+  });
+
+  test('ten days, today last; ten weeks from Monday; twelve months', () {
+    final days = studyStatistics([], now: now, span: StatisticsSpan.days);
+    expect(days.periods, hasLength(10));
+    expect(days.periods.first.start, DateTime(2026, 10, 3));
+    expect(days.periods.last.start, DateTime(2026, 10, 12));
+    expect(days.studied, false);
+
+    final weeks = studyStatistics([], now: now, span: StatisticsSpan.weeks);
+    expect(weeks.periods, hasLength(10));
+    expect(weeks.periods.last.start, DateTime(2026, 10, 12));
+    expect(weeks.periods.first.start, DateTime(2026, 8, 10));
+
+    final months = studyStatistics([], now: now, span: StatisticsSpan.months);
+    expect(months.periods, hasLength(12));
+    expect(months.periods.first.start, DateTime(2025, 11));
+    expect(months.periods.last.start, DateTime(2026, 10));
+  });
+
+  test("each period holds its reviews' time and ratings; older ones count "
+      'in the totals only', () {
+    final statistics = studyStatistics(
+      [
+        review(DateTime(2026, 10, 6, 9), rating: Rating.again),
+        review(DateTime(2026, 10, 11, 23), rating: Rating.easy),
+        review(DateTime(2026, 10, 12, 8), rating: Rating.easy),
+        review(DateTime(2026, 7, 1, 8)),
+      ],
+      now: now,
+      span: StatisticsSpan.weeks,
+    );
+
+    final [...earlier, lastWeek, thisWeek] = statistics.periods;
+    expect(thisWeek.ratings, {Rating.easy: 1});
+    expect(lastWeek.ratings, {Rating.again: 1, Rating.easy: 1});
+    expect(lastWeek.time, const Duration(seconds: 40));
+    expect(earlier.every((period) => period.reviews == 0), true);
+    expect(statistics.total, const Duration(seconds: 80));
+  });
+}