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/home/model/study_activity_test.dart

@@ -0,0 +1,128 @@
+import 'package:flutter_test/flutter_test.dart';
+import 'package:halkyon_learn_flutter/src/feature/home/model/study_activity.dart';
+import 'package:halkyon_learn_flutter/src/feature/home/model/study_goal.dart';
+
+void main() {
+  // A Wednesday, in the afternoon, local time.
+  final now = DateTime(2026, 10, 7, 15);
+  DateTime day(int d) => DateTime(2026, 10, d);
+  DateTime at(int d, [int hour = 10]) => DateTime(2026, 10, d, hour);
+
+  StudyActivity studied(List<int> days) =>
+      StudyActivity.of(ratings: [for (final d in days) at(d)], written: []);
+
+  test('counts ratings and cards written by the local day they fell on', () {
+    final activity = StudyActivity.of(
+      ratings: [at(5, 9), at(5, 23)],
+      written: [at(6)],
+    );
+
+    expect(activity.on(day(5)), 2);
+    expect(activity.on(day(6)), 1);
+    expect(activity.studiedOn(day(4)), isFalse);
+  });
+
+  test('starts on the first day anything happened, imports included, which '
+      "don't count as studying", () {
+    final activity = StudyActivity.of(
+      ratings: [at(6)],
+      written: [at(5)],
+      imported: [at(4, 23)],
+    );
+
+    expect(activity.start, day(4));
+    expect(activity.studiedOn(day(4)), isFalse);
+    expect(StudyActivity.none.start, isNull);
+  });
+
+  group('The streak', () {
+    test('runs up to today once studied today', () {
+      expect(studyStreak(studied([5, 6, 7]), {}, now), 3);
+    });
+
+    test('runs up to yesterday while today is still open', () {
+      expect(studyStreak(studied([5, 6]), {}, now), 2);
+    });
+
+    test('is broken by a day without studying or resting', () {
+      expect(studyStreak(studied([4, 6, 7]), {}, now), 2);
+      expect(studyStreak(studied([4, 5]), {}, now), 0);
+    });
+
+    test('keeps going over a rest day, which counts', () {
+      expect(studyStreak(studied([4, 6, 7]), {day(5)}, now), 4);
+    });
+  });
+
+  test('a week runs Monday to Sunday, each day as it stands', () {
+    final week = weekOf(day(7), studied([5, 7]), {day(6)}, now);
+
+    expect(week.map((each) => each.day.day), [5, 6, 7, 8, 9, 10, 11]);
+    expect(week.map((each) => each.state), [
+      DayState.studied,
+      DayState.rested,
+      DayState.studied,
+      DayState.upcoming,
+      DayState.upcoming,
+      DayState.upcoming,
+      DayState.upcoming,
+    ]);
+    expect(
+      weekOf(day(7), studied([5]), {}, now).take(3).map((each) => each.state),
+      [DayState.studied, DayState.missed, DayState.today],
+    );
+  });
+
+  test('days before the first activity have nothing to miss; a rest day '
+      'still shows', () {
+    expect(
+      weekOf(day(7), studied([6]), {}, now).take(3).map((each) => each.state),
+      [DayState.notStarted, DayState.studied, DayState.today],
+    );
+    expect(
+      weekOf(day(7), studied([]), {day(5)}, now).take(3).map((e) => e.state),
+      [DayState.rested, DayState.notStarted, DayState.today],
+    );
+  });
+
+  test('heat starts at one card and darkens in coarse steps', () {
+    expect([0, 1, 4, 5, 9, 10, 19, 20].map(heatLevel), [
+      0,
+      1,
+      1,
+      2,
+      2,
+      3,
+      3,
+      4,
+    ]);
+  });
+
+  test('cards fall due on their day; overdue ones today', () {
+    expect(dueByDay([at(1), at(7), at(9), at(9, 20)], day(7)), {
+      day(7): 2,
+      day(9): 2,
+    });
+  });
+
+  test("a goal's progress counts the ratings of its day or week", () {
+    final ratings = [at(4), at(5), at(6), at(7), at(7, 14)];
+
+    expect(
+      goalProgress(
+        const StudyGoal(period: GoalPeriod.day, target: 20),
+        ratings,
+        now,
+      ),
+      2,
+    );
+    expect(
+      goalProgress(
+        const StudyGoal(period: GoalPeriod.week, target: 20),
+        ratings,
+        now,
+      ),
+      4,
+    );
+  });
+}