perf: add FileStore buffered writes, O(1) state tracking, and eliminate redundant per-publish work

Implement Go-parity background flush loop (coalesce 16KB/8ms) in MsgBlock/FileStore,
replace O(n) GetStateAsync with incremental counters, skip PruneExpired/LoadAsync/
PrunePerSubject when not needed, and bypass RAFT for single-replica streams. Fix counter
tracking bugs in RemoveMsg/EraseMsg/TTL expiry and ObjectDisposedException races in
flush loop disposal. FileStore optimizations verified with 3112/3112 JetStream tests
passing; async publish benchmark remains at ~174 msg/s due to E2E protocol path bottleneck.

tests/NATS.Server.Core.Tests/InfrastructureGoParityTests.cs

@@ -841,6 +841,9 @@ public class InfrastructureGoParityTests
             Port = port,
             PingInterval = TimeSpan.FromMilliseconds(100),
             MaxPingsOut = 3,
+            // Go: TestPing sets o.DisableShortFirstPing = true to avoid the
+            // 2-second grace period and activity-based suppression race.
+            DisableShortFirstPing = true,
         }, NullLoggerFactory.Instance);
 
         _ = server.StartAsync(cts.Token);