feat(ui): the question mark's explanation offers a link beside OK

HelpButton takes an action, a label and what it does, shown beside OK where
there is more to read elsewhere, as what a map's source keeps (Train's ADR
0030, D63).

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

@@ -26,4 +26,22 @@ void main() {
     await tester.pumpAndSettle();
     expect(find.text('How sources are cited.'), findsNothing);
   });
+  testWidgets('an action offered stands beside OK', (tester) async {
+    var read = false;
+    await tester.pumpWidget(
+      testApp(
+        Scaffold(
+          body: HelpButton(
+            'Map by OpenFreeMap.',
+            action: (label: 'What it keeps', onPressed: () => read = true),
+          ),
+        ),
+      ),
+    );
+    await tester.tap(find.byType(HelpButton));
+    await tester.pumpAndSettle();
+    await tester.tap(find.text('What it keeps'));
+    expect(read, isTrue);
+    expect(find.text('OK'), findsOneWidget);
+  });
 }