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/note/model/note_merge_test.dart

@@ -0,0 +1,80 @@
+import 'package:flutter_test/flutter_test.dart';
+import 'package:halkyon_learn_flutter/src/core/content/rich_content.dart';
+import 'package:halkyon_learn_flutter/src/feature/note/model/note_content.dart';
+import 'package:halkyon_learn_flutter/src/feature/note/model/note_merge.dart';
+import 'package:halkyon_learn_flutter/src/feature/note/model/note_revision.dart';
+
+/// Rich content holding just [text], enough to tell versions apart.
+RichContent text(String text) => RichContent({
+  'document': {'type': 'page', 'note': text},
+});
+
+NoteContent note(String title, String body) =>
+    NoteContent(title: title, body: text(body));
+
+/// A revision with the id [id] (ids sort by creation, like UUID v7).
+NoteRevision revision(
+  String id,
+  NoteContent content, {
+  Set<String> parents = const {},
+}) => NoteRevision(id: id, parents: parents, content: content);
+
+void main() {
+  test('the newest revision of a chain is the current one', () {
+    final merged = mergeNoteRevisions([
+      revision('r1', note('Particles', 'は marks the topic')),
+      revision('r2', note('Particles', 'は and が'), parents: {'r1'}),
+    ])!;
+
+    expect(merged.content, note('Particles', 'は and が'));
+    expect(merged.heads, {'r2'});
+    expect(merged.conflicts, isEmpty);
+  });
+
+  test('a title changed on one device and the body on another both '
+      'survive', () {
+    final merged = mergeNoteRevisions([
+      revision('r1', note('Particles', 'は marks the topic')),
+      revision('r2', note('は and が', 'は marks the topic'), parents: {'r1'}),
+      revision('r3', note('Particles', 'は and が'), parents: {'r1'}),
+    ])!;
+
+    expect(merged.content, note('は and が', 'は and が'));
+    expect(merged.conflicts, isEmpty);
+  });
+
+  test('a body changed differently on two devices is a conflict with both '
+      'versions, the newest showing', () {
+    final merged = mergeNoteRevisions([
+      revision('r1', note('Particles', 'は marks the topic')),
+      revision('r2', note('Particles', 'は: topic'), parents: {'r1'}),
+      revision('r3', note('Particles', 'が: subject'), parents: {'r1'}),
+    ])!;
+
+    expect(merged.conflicts, {
+      NoteField.body: [text('は: topic'), text('が: subject')],
+    });
+    expect(merged.content.body, text('が: subject'));
+    expect(merged.heads, {'r2', 'r3'});
+  });
+
+  test('without revisions there is no note', () {
+    expect(mergeNoteRevisions(const []), isNull);
+  });
+
+  group('NoteContent', () {
+    test('is blank with neither title nor text', () {
+      expect(NoteContent.empty().isBlank, isTrue);
+      expect(note('  ', '').isBlank, isTrue);
+      expect(note('Particles', '').isBlank, isFalse);
+      expect(
+        NoteContent(body: RichContent.plain('は')).isBlank,
+        isFalse,
+      );
+    });
+
+    test('is stored with its title trimmed', () {
+      expect(note('  Particles ', 'は').normalized, note('Particles', 'は'));
+    });
+  });
+}