feat(ui): a question mark that keeps a section's explanation one tap away

HelpButton, beside a section's or a setting's title: the explanation opens
in a small dialog, under its title where given, rather than standing in the
everyday view (the developer's review of 3 Nov 2026). FieldInfoButton's
tooltip stays for a field's short hint.

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

packages/halkyon_ui/test/widget/help_button_test.dart

@@ -0,0 +1,29 @@
+import 'package:flutter_test/flutter_test.dart';
+import 'package:halkyon_ui/halkyon_ui.dart';
+import 'package:material_ui/material_ui.dart';
+
+import '../support/test_app.dart';
+
+void main() {
+  testWidgets('the explanation stays hidden until the question mark is '
+      'tapped, then shows under its title and closes again', (tester) async {
+    await tester.pumpWidget(
+      testApp(
+        const Scaffold(
+          body: HelpButton('How sources are cited.', title: 'Citation style'),
+        ),
+      ),
+    );
+    expect(find.text('How sources are cited.'), findsNothing);
+    expect(find.byTooltip("What's this?"), findsOneWidget);
+
+    await tester.tap(find.byType(HelpButton));
+    await tester.pumpAndSettle();
+    expect(find.text('Citation style'), findsOneWidget);
+    expect(find.text('How sources are cited.'), findsOneWidget);
+
+    await tester.tap(find.text('OK'));
+    await tester.pumpAndSettle();
+    expect(find.text('How sources are cited.'), findsNothing);
+  });
+}