refactor(app): studysets and settings are written in the newest schema

Decided 2026-10-07: the codecs write the newest schema every time, as
before, rather than the oldest one that holds what is there. An older
app loses sight of a studyset or of the settings once a newer one has
written them, until it is updated too.

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

halkyon_learn_flutter/lib/src/feature/settings/data/settings_payload.dart

@@ -19,22 +19,17 @@ import 'package:halkyon_learn_flutter/src/feature/tag/model/tag.dart';
 // layout (the widgets' order and those hidden, by name). Schema 4 added the
 // Pomodoro timer's profiles and the one chosen, left out while as they come.
 // Older schemas read with what they lack as it comes.
-//
-// Settings whose Pomodoro profiles are as they come are still written in
-// schema 3: an app that knows only schema 3 skips a schema 4 payload whole,
-// and would lose every setting.
 
-/// The newest schema this app writes.
+/// The schema this app writes.
 const settingsSchemaVersion = 4;
 
-/// Encodes [settings] into their payload, in the oldest schema that holds
-/// them.
+/// Encodes [settings] into their payload.
 ByteData encodeSettingsPayload(PersonSettings settings) {
   const defaults = StudyOptions();
   final study = settings.study;
   final pomodoro = settings.pomodoro;
   return encodePayload(
-    pomodoro == const PomodoroSettings() ? 3 : settingsSchemaVersion,
+    settingsSchemaVersion,
     {
       'studyTags': {
         for (final MapEntry(key: studysetId, value: tags)

halkyon_learn_flutter/lib/src/feature/studyset/data/studyset_payload.dart

@@ -13,18 +13,13 @@ import 'package:halkyon_learn_flutter/src/feature/studyset/model/studyset_conten
 // without them. Schema 3 added the study options (`study`: new cards and
 // reviews a day, 0 for no limit, and the order), left out when the studyset
 // has none.
-//
-// A studyset without study options is still written in schema 2: an app that
-// knows only schema 2 skips a schema 3 payload whole, and would lose sight of
-// every studyset edited on a newer app otherwise.
 
-/// The newest schema this app writes.
+/// The schema this app writes.
 const studysetSchemaVersion = 3;
 
-/// Encodes a studyset's content into its payload, in the oldest schema that
-/// holds it.
+/// Encodes a studyset's content into its payload.
 ByteData encodeStudysetPayload(StudysetContent content) => encodePayload(
-  content.studyOptions == null ? 2 : studysetSchemaVersion,
+  studysetSchemaVersion,
   {
     'title': content.title,
     'description': content.description.toJson(),

halkyon_learn_flutter/test/feature/settings/model/person_settings_test.dart

@@ -181,11 +181,14 @@ void main() {
       expect(decodeSettingsPayload(payload), settings);
     });
 
-    test('as they come, are written in the third schema, which older apps '
-        'read', () {
-      final payload = encodeSettingsPayload(const PersonSettings());
+    test('as they come, are left out, in the fourth schema all the same', () {
+      final payload = decodePayload(
+        encodeSettingsPayload(const PersonSettings()),
+        newestSchema: 4,
+      );
 
-      expect(decodePayload(payload, newestSchema: 4)['schemaVersion'], 3);
+      expect(payload['schemaVersion'], 4);
+      expect(payload.keys, isNot(contains('pomodoro')));
     });
 
     test('that are malformed read as they come, keeping the other '

halkyon_learn_flutter/test/feature/studyset/data/studyset_payload_test.dart

@@ -70,13 +70,13 @@ void main() {
       expect(json(payload)['schemaVersion'], studysetSchemaVersion);
     });
 
-    test('without study options still writes the second schema, which '
-        'older apps read', () {
+    test('without study options writes the third schema all the same', () {
       final payload = encodeStudysetPayload(
         StudysetContent(title: 'Japanese', description: RichContent.empty()),
       );
 
-      expect(json(payload)['schemaVersion'], 2);
+      expect(json(payload)['schemaVersion'], 3);
+      expect(json(payload).keys, isNot(contains('study')));
     });
 
     test('keeps the study options', () {