feat: execute post-baseline jetstream parity plan

tests/NATS.Server.Tests/JetStreamFileStoreBlockParityTests.cs

@@ -0,0 +1,30 @@
+using NATS.Server.JetStream.Storage;
+using System.Text;
+
+namespace NATS.Server.Tests;
+
+public class JetStreamFileStoreBlockParityTests
+{
+    [Fact]
+    public async Task File_store_rolls_blocks_and_recovers_index_without_full_file_rewrite()
+    {
+        var dir = Path.Combine(Path.GetTempPath(), $"nats-js-filestore-block-{Guid.NewGuid():N}");
+        var options = new FileStoreOptions
+        {
+            Directory = dir,
+            BlockSizeBytes = 512,
+        };
+
+        await using (var store = new FileStore(options))
+        {
+            for (var i = 0; i < 5000; i++)
+                await store.AppendAsync("orders.created", Encoding.UTF8.GetBytes($"payload-{i}"), default);
+
+            store.BlockCount.ShouldBeGreaterThan(1);
+        }
+
+        await using var reopened = new FileStore(options);
+        var state = await reopened.GetStateAsync(default);
+        state.Messages.ShouldBe((ulong)5000);
+    }
+}