Sync: mirror a pulled page in one write and read the outbox once per page, not twice per change

tests/DodoSSH.Client.Storage.Tests/CacheStoreTests.cs

@@ -128,6 +128,42 @@ public sealed class CacheStoreTests : IAsyncLifetime
         items.ShouldHaveSingleItem().Version.ShouldBe(2);
     }
 
+    /// <remarks>
+    /// A pull's page in one write. What it leaves has to be exactly what the same items saved one at a time
+    /// would have left: rows that existed replaced, new ones created, and an item given twice ending at the
+    /// later of the two — a page can carry several changes to the same item.
+    /// </remarks>
+    [Fact]
+    public async Task SavingMany_LeavesWhatSavingThemOneAtATimeWould()
+    {
+        var existing = Guid.CreateVersion7();
+        var fresh = Guid.CreateVersion7();
+        var twice = Guid.CreateVersion7();
+
+        await harness.Items.SaveAsync(Item(existing, version: 1, seed: 1), Token);
+
+        await harness.Items.SaveManyAsync(
+            [
+                Item(existing, version: 2, changeSequence: 2, seed: 2),
+                Item(fresh, version: 1, changeSequence: 3, seed: 3),
+                Item(twice, version: 1, changeSequence: 4, seed: 4),
+                Item(twice, version: 2, changeSequence: 5, deleted: true),
+            ],
+            Token);
+
+        (await harness.Items.FindAsync(VaultId, SyncEntityType.Host, existing, Token))
+            .ShouldNotBeNull().Version.ShouldBe(2);
+
+        (await harness.Items.FindAsync(VaultId, SyncEntityType.Host, fresh, Token))
+            .ShouldNotBeNull().Version.ShouldBe(1);
+
+        var last = (await harness.Items.FindAsync(VaultId, SyncEntityType.Host, twice, Token)).ShouldNotBeNull();
+        last.Version.ShouldBe(2);
+        last.IsDeleted.ShouldBeTrue();
+
+        (await harness.Items.ListAsync(VaultId, SyncEntityType.Host, true, Token)).Count.ShouldBe(3);
+    }
+
     [Fact]
     public async Task ATombstone_IsHiddenFromTheListButStillFindable()
     {