feat(todo): a task's sub-tasks held and dragged into another order in its sheet

As a project's rows are (D16): a long press on a sub-task in the task's
sheet or pane lifts it, and dropped on the upper or lower half of
another it goes there, still the task's sub-task (the developer's
review, 10 Nov 2026).

Whats-New: In a task's details, hold a sub-task to drag it into another order.
Whats-New-De: In den Details einer Aufgabe hältst du eine Unteraufgabe gedrückt, um sie in eine andere Reihenfolge zu ziehen.

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/todo/halkyon_todo_flutter/test/app/todo_app_test.dart

@@ -5614,6 +5614,53 @@ void main() {
                 .compareTo(tester.getCenter(row(b)).dy),
           );
 
+      testWidgets("in a task's sheet, a sub-task held and dropped on the "
+          'upper half of another goes before it, still its sub-task (the '
+          "developer's review, 10 Nov 2026)", (tester) async {
+        await start(tester, usedBefore: true);
+        final pack = (await tester.runAsync(
+          () => tasks().add(TaskContent(title: 'Pack', projectId: inbox())),
+        ))!;
+        for (final (index, title) in ['Alpha', 'Bravo', 'Charlie'].indexed) {
+          await seed(
+            tester,
+            TaskContent(
+              title: title,
+              projectId: inbox(),
+              parentId: pack,
+              position: index.toDouble(),
+            ),
+          );
+        }
+        await goTab(tester, 'Inbox');
+        await tester.tap(find.text('Pack'));
+        await tester.pumpAndSettle();
+        await tester.drag(
+          find.byKey(taskSheetHandleKey),
+          const Offset(0, -700),
+        );
+        await tester.pumpAndSettle();
+        Finder inSheet(String title) => find.descendant(
+          of: find.byType(DraggableScrollableSheet),
+          matching: row(title),
+        );
+
+        await dragFrom(
+          tester,
+          tester.getCenter(inSheet('Charlie')),
+          tester.getTopLeft(inSheet('Alpha')) + const Offset(100, 6),
+        );
+
+        final after = await stored(tester);
+        expect(after['Charlie']!.position, lessThan(after['Alpha']!.position));
+        expect(after['Charlie']!.parentId, pack);
+        final ys = [
+          for (final title in ['Charlie', 'Alpha', 'Bravo'])
+            tester.getCenter(inSheet(title)).dy,
+        ];
+        expect(ys, orderedEquals([...ys]..sort()));
+      });
+
       testWidgets('a row held and dropped on the upper half of another goes '
           'before it, which can be undone', (tester) async {
         await inboxOf(tester, ['Alpha', 'Bravo', 'Charlie']);