feat(todo): a task holds reminders at a time of their own, and keeps the fields it doesn't know

ReminderTime is a day and minutes after midnight, the wall clock without
a zone as TaskDate's; a task's reminderTimes are written as
[{"day", "time"}] and left out where there are none, so older payloads
read with none. The payload's schema stays 1 (as D17's view): what a
newer version wrote at its top that this one doesn't read is kept in
TaskContent.unknown and written back beneath the fields it knows, so a
task changed here keeps it (ADR 0033, D18).

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/todo/halkyon_todo_flutter/test/feature/task/task_payload_test.dart

@@ -43,6 +43,10 @@ void main() {
     priority: Priority.p2,
     labelIds: const ['l1', 'l2'],
     reminders: const [30, 0],
+    reminderTimes: const [
+      ReminderTime(Day(2026, 11, 1), 20 * 60),
+      ReminderTime(Day(2026, 11, 2), 6 * 60),
+    ],
     completedAt: DateTime(2026, 11, 2, 7),
     completions: [DateTime(2026, 10, 19, 7, 5), DateTime(2026, 10, 22, 6)],
     hideDoneSubTasks: true,
@@ -80,6 +84,10 @@ void main() {
         'priority': 2,
         'labelIds': ['l1', 'l2'],
         'reminders': [30, 0],
+        'reminderTimes': [
+          {'day': '2026-11-01', 'time': 1200},
+          {'day': '2026-11-02', 'time': 360},
+        ],
         'completedAt': DateTime(2026, 11, 2, 7).toUtc().toIso8601String(),
         'completions': [
           DateTime(2026, 10, 19, 7, 5).toUtc().toIso8601String(),
@@ -136,6 +144,16 @@ void main() {
             'date': {'day': '2026-11-02', 'repeat': repeat},
           },
         {...base, 'date': 'today'},
+        for (final at in [
+          {'day': '2026-11-02'},
+          {'day': 'soon', 'time': 60},
+          {'day': '2026-11-02', 'time': 1440},
+          '2026-11-02 13:00',
+        ])
+          {
+            ...base,
+            'reminderTimes': [at],
+          },
       ]) {
         expect(decodeTask(json(broken)), isNull, reason: '$broken');
       }
@@ -159,6 +177,55 @@ void main() {
       }
     });
 
+    test('of the schema before reminders at a time of their own reads with '
+        'none (D18)', () {
+      final older = decodeTask(
+        json({
+          'schemaVersion': 1,
+          'title': 'A',
+          'projectId': 'p',
+          'date': {'day': '2026-11-02', 'time': 540},
+          'reminders': [30],
+        }),
+      );
+      expect(older?.reminders, [30]);
+      expect(older?.reminderTimes, isEmpty);
+      expect(older?.unknown, isEmpty);
+    });
+
+    test('what a newer version wrote that this one doesn’t know is kept, '
+        'and written back as it was, however the task changes here (D18)', () {
+      final newer = {
+        'schemaVersion': 1,
+        'title': 'A',
+        'projectId': 'p',
+        'location': {'lat': 52.5, 'lon': 13.4},
+        'note': 'kept',
+      };
+      final read = decodeTask(json(newer))!;
+      expect(read.unknown, {
+        'location': {'lat': 52.5, 'lon': 13.4},
+        'note': 'kept',
+      });
+      final renamed = read.copyWith(title: 'B');
+      expect(written(encodeTask(renamed)), {
+        ...newer,
+        'title': 'B',
+        'position': 0.0,
+      });
+      expect(decodeTask(encodeTask(renamed)), renamed);
+    });
+
+    test('a field it knows wins over one kept as unknown of the same name', () {
+      const task = TaskContent(
+        title: 'A',
+        projectId: 'p',
+        unknown: {'title': 'stale', 'extra': 1},
+      );
+      expect(written(encodeTask(task))['title'], 'A');
+      expect(written(encodeTask(task))['extra'], 1);
+    });
+
     test('a priority it doesn’t know is none', () {
       expect(
         decodeTask(