feat(learn): spelling and grammar checked as people write (ADR 0024)

Where the instance's LanguageTool checks the language, the editors
underline what it finds after a pause, a paragraph at a time, a tap
offering its suggestions (GrammarHost, GrammarCheckController,
GrammarOverlay; nothing written into the document). A card's front is
checked in the studyset's front language, its back and answers in the
back's, everything else in the language recognised.

In a private studyset nothing is sent until the person agrees for it,
asked once in a banner above the editor and kept in their settings
(an agreement under privateAgreements, a refusal in grammarDeclined,
settings schema 14), taken back in the studyset's privacy dialog.
Settings > Writing turns it off on the device.

The field's Check asks LanguageTool first where it checks the
studyset's language, in a sheet offering to ask the assistant why;
elsewhere it asks the assistant as before (amends ADR 0021, D54).

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/settings/model/person_settings_test.dart

@@ -692,4 +692,82 @@ void main() {
       expect(both.foldedWith(both), both);
     });
   });
+
+  group('checking what is written in a private studyset (ADR 0024)', () {
+    test('is asked until answered; agreed, it is an agreement as an '
+        "assistant's, declined it is declined, and taken back it is asked "
+        'again', () {
+      const settings = PersonSettings();
+      expect(settings.grammarAnswerOf('ja'), isNull);
+
+      final agreed = settings.withGrammarAnswer('ja', check: true);
+      expect(agreed.grammarAnswerOf('ja'), isTrue);
+      expect(agreed.agreedToSend('ja', grammarAgreementKey), isTrue);
+      expect(agreed.grammarAnswerOf('ko'), isNull);
+
+      final declined = agreed.withGrammarAnswer('ja', check: false);
+      expect(declined.grammarAnswerOf('ja'), isFalse);
+      expect(
+        declined.agreedToSend('ja', grammarAgreementKey),
+        isFalse,
+        reason: 'one answer at a time',
+      );
+
+      expect(declined.withoutGrammarAnswer('ja').grammarAnswerOf('ja'), isNull);
+      expect(agreed.withoutGrammarAnswer('ja').grammarAnswerOf('ja'), isNull);
+    });
+
+    test('survives a round trip, in the fourteenth schema, the declined '
+        'left out while there are none; a payload from before declined '
+        'none', () {
+      final settings = const PersonSettings()
+          .withGrammarAnswer('ja', check: true)
+          .withGrammarAnswer('ko', check: false)
+          .withGrammarAnswer('de', check: false);
+
+      expect(settingsSchemaVersion, greaterThanOrEqualTo(14));
+      final read = decodeSettingsPayload(encodeSettingsPayload(settings));
+      expect(read, settings);
+      expect(read.grammarAnswerOf('ja'), isTrue);
+      expect(read.grammarAnswerOf('ko'), isFalse);
+      expect(
+        decodePayload(
+          encodeSettingsPayload(const PersonSettings()),
+          newestSchema: settingsSchemaVersion,
+        ).keys,
+        isNot(contains('grammarDeclined')),
+      );
+      expect(
+        decodeSettingsPayload(
+          encodePayload(13, {'studyTags': <String, Object?>{}}),
+        ).grammarDeclined,
+        isEmpty,
+      );
+    });
+
+    test("is checked, as two devices' settings fold, only where both "
+        'agreed, and declined where either declined: an older row never '
+        'sends what was declined since', () {
+      final agreed = const PersonSettings().withGrammarAnswer(
+        'ja',
+        check: true,
+      );
+      final declined = const PersonSettings().withGrammarAnswer(
+        'ja',
+        check: false,
+      );
+
+      for (final folded in [
+        agreed.foldedWith(declined),
+        declined.foldedWith(agreed),
+      ]) {
+        expect(folded.grammarAnswerOf('ja'), isFalse);
+      }
+      expect(
+        agreed.foldedWith(const PersonSettings()).grammarAnswerOf('ja'),
+        isNull,
+        reason: 'a row not agreeing asks again',
+      );
+    });
+  });
 }