feat: implement jetstream memstore core behavior

tests/NATS.Server.Tests/MemStoreTests.cs

@@ -0,0 +1,20 @@
+using NATS.Server.JetStream.Storage;
+
+namespace NATS.Server.Tests;
+
+public class MemStoreTests
+{
+    [Fact]
+    public async Task MemStore_supports_append_load_and_purge()
+    {
+        var store = new MemStore();
+        var seq1 = await store.AppendAsync("a", "one"u8.ToArray(), default);
+        var seq2 = await store.AppendAsync("a", "two"u8.ToArray(), default);
+
+        seq2.ShouldBe(seq1 + 1);
+        (await store.LoadAsync(seq2, default))!.Payload.Span.SequenceEqual("two"u8).ShouldBeTrue();
+
+        await store.PurgeAsync(default);
+        (await store.GetStateAsync(default)).Messages.ShouldBe((ulong)0);
+    }
+}