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.

src/NATS.Server/NatsOptions.cs

@@ -25,6 +25,11 @@ public sealed class NatsOptions
     public TimeSpan PingInterval { get; set; } = NatsProtocol.DefaultPingInterval;
     public int MaxPingsOut { get; set; } = NatsProtocol.DefaultPingMaxOut;
 
+    // Go: opts.go — DisableShortFirstPing. When true, the first PING timer tick
+    // is not suppressed by the FirstPongSent / 2-second grace period.
+    // Useful in tests to ensure deterministic ping behavior.
+    public bool DisableShortFirstPing { get; set; }
+
     // Subscription limits
     public int MaxSubs { get; set; }             // 0 = unlimited (per-connection)
     public int MaxSubTokens { get; set; }        // 0 = unlimited