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.JetStream.Tests/JetStream/Cluster/JetStreamClusterGoParityTests.cs

@@ -533,7 +533,7 @@ public class JetStreamClusterGoParityTests
 
     // Go reference: TestJetStreamClusterMetaSyncOrphanCleanup — meta state clean after stream delete
     // Skip: delete API handler doesn't yet propagate to meta group
-    [Fact(Skip = "Stream delete API handler does not yet call ProposeDeleteStreamAsync on meta group")]
+    [Fact]
     public async Task Meta_state_does_not_track_deleted_streams()
     {
         await using var cluster = await JetStreamClusterFixture.StartAsync(3);