feat: add atomic file writer with SemaphoreSlim for crash-safe state writes (Gap 1.6)

- Add AtomicFileWriter static helper: writes to {path}.{random}.tmp, flushes,
  then File.Move(overwrite:true) — concurrent-safe via unique temp path per call
- Add _stateWriteLock (SemaphoreSlim 1,1) to FileStore; dispose in both Dispose
  and DisposeAsync paths
- Promote WriteStreamState to async WriteStreamStateAsync using AtomicFileWriter
  under the write lock; FlushAllPending now returns Task
- Update IStreamStore.FlushAllPending signature to Task; fix MemStore no-op impl
- Fix FileStoreCrashRecoveryTests to await FlushAllPending (3 sync→async tests)
- Add 9 AtomicFileWriterTests covering create, no-tmp-remains, overwrite,
  concurrent safety, memory overload, empty data, and large payload

tests/NATS.Server.Tests/JetStream/Storage/FileStoreCrashRecoveryTests.cs

@@ -54,7 +54,7 @@ public sealed class FileStoreCrashRecoveryTests : IDisposable
     // Go: TestFileStoreSyncIntervals (filestore_test.go) — verifies that pending writes
     // are flushed to disk and the .blk file is non-empty after FlushAllPending.
     [Fact]
-    public void FlushAllPending_flushes_active_block()
+    public async Task FlushAllPending_flushes_active_block()
     {
         // Arrange: write a message to the store.
         const string sub = "flush-block";
@@ -64,7 +64,7 @@ public sealed class FileStoreCrashRecoveryTests : IDisposable
         store.StoreMsg("events.a", null, "payload-for-flush"u8.ToArray(), 0L);
 
         // Act: flush all pending writes.
-        store.FlushAllPending();
+        await store.FlushAllPending();
 
         // Assert: at least one .blk file exists and it is non-empty, proving the
         // active block was flushed to disk.
@@ -81,7 +81,7 @@ public sealed class FileStoreCrashRecoveryTests : IDisposable
     // Go: TestFileStoreWriteFullStateBasics (filestore_test.go:5461) — verifies that
     // WriteStreamState creates a valid, atomic stream.state checkpoint file.
     [Fact]
-    public void FlushAllPending_writes_stream_state_file()
+    public async Task FlushAllPending_writes_stream_state_file()
     {
         // Arrange: store several messages across subjects.
         const string sub = "state-file";
@@ -93,13 +93,13 @@ public sealed class FileStoreCrashRecoveryTests : IDisposable
         store.StoreMsg("events.a",   null, "event-1"u8.ToArray(), 0L);
 
         // Act: flush — this should write stream.state atomically.
-        store.FlushAllPending();
+        await store.FlushAllPending();
 
         // Assert: stream.state exists and no leftover .tmp file.
+        // AtomicFileWriter uses {path}.{random}.tmp so check by extension, not exact name.
         var statePath = Path.Combine(dir, "stream.state");
-        var tmpPath   = statePath + ".tmp";
         File.Exists(statePath).ShouldBeTrue("stream.state checkpoint must exist after FlushAllPending");
-        File.Exists(tmpPath).ShouldBeFalse("stream.state.tmp must be renamed away after atomic write");
+        Directory.GetFiles(dir, "*.tmp").ShouldBeEmpty("all .tmp staging files must be renamed away after atomic write");
 
         // Assert: the file is valid JSON with the expected fields.
         var json = File.ReadAllText(statePath);
@@ -118,17 +118,17 @@ public sealed class FileStoreCrashRecoveryTests : IDisposable
     // Go: FlushAllPending is idempotent — calling it twice must not throw and must
     // overwrite the previous state file with the latest state.
     [Fact]
-    public void FlushAllPending_is_idempotent()
+    public async Task FlushAllPending_is_idempotent()
     {
         const string sub = "flush-idempotent";
         var dir = StoreDir(sub);
         using var store = CreateStore(sub);
 
         store.StoreMsg("foo", null, "msg-1"u8.ToArray(), 0L);
-        store.FlushAllPending();
+        await store.FlushAllPending();
 
         store.StoreMsg("foo", null, "msg-2"u8.ToArray(), 0L);
-        store.FlushAllPending();
+        await store.FlushAllPending();
 
         // The state file should reflect the second flush (2 messages, seq 1..2).
         var statePath = Path.Combine(dir, "stream.state");
@@ -197,7 +197,7 @@ public sealed class FileStoreCrashRecoveryTests : IDisposable
                 store.StoreMsg("events", null, Encoding.UTF8.GetBytes($"msg-{i}"), 0L);
 
             // Flush to ensure data is on disk before we close.
-            store.FlushAllPending();
+            await store.FlushAllPending();
         }
 
         // Simulate a crash mid-write by truncating the .blk file by a few bytes at