perf(sync): the fork's signed commit with fast merged deletes and batched inserts

The offline sync package's shared part now comes from the fork's branch
halkyon/batch-inserts (949a664, signed), adding two changes to the pin.
A merge only deleting rows skips the projection pass when every visible
row below them goes with them (a card deleted on one device, in a
library of 8,000 cards: 5.4 s for the server to merge, now about 35 ms;
4 s on each other device, now about 0.1 s). And a merge the pass leaves
out writes its new rows up to 1,000 to a statement, its proof also
taking nested studysets (a first push of that library: 35 to 41 s on
the server, now about 5; a device receiving it, 60 s, now 44).

A new sync test deletes a card on one device while another rates it:
the merged delete declines the shortcut there, and both devices end
without the card or its rating.

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

halkyon_learn_flutter/test/sync/card_sync_test.dart

@@ -158,6 +158,27 @@ void main() {
     expect(scheduleOf(id, onPhone), scheduleOf(id, onLaptop));
   });
 
+  test('a card deleted on one device takes the ratings another made of it '
+      'meanwhile with it, on both', () async {
+    final phone = await device('phone');
+    final laptop = await device('laptop');
+    final (studyset, id) = await shared(phone, laptop);
+
+    await phone.cards.delete((await phone.card(studyset, id))!);
+    await laptop.reviews.record(
+      cardId: id,
+      rating: Rating.good,
+      reviewedAt: DateTime.utc(2026, 10, 5, 8),
+      duration: const Duration(seconds: 3),
+    );
+    await syncBoth(phone, laptop);
+
+    for (final device in [phone, laptop]) {
+      expect(await device.card(studyset, id), isNull);
+      expect(await device.reviews.reviewsOf({id}), isEmpty);
+    }
+  });
+
   test("a word's reading changed on one device and its meanings on another "
       'both survive', () async {
     final phone = await device('phone');