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/Configuration/ConfigProcessor.cs

@@ -1750,12 +1750,13 @@ public static class ConfigProcessor
 
     // ─── Type conversion helpers ───────────────────────────────────
 
+    // Go: opts.go — strconv.Atoi after strings.TrimSuffix(s, "%") for sampling values.
     private static int ToInt(object? value) => value switch
     {
         long l => (int)l,
         int i => i,
         double d => (int)d,
-        string s when int.TryParse(s, NumberStyles.Integer, CultureInfo.InvariantCulture, out var i) => i,
+        string s when int.TryParse(s.AsSpan().TrimEnd('%'), NumberStyles.Integer, CultureInfo.InvariantCulture, out var i) => i,
         _ => throw new FormatException($"Cannot convert {value?.GetType().Name ?? "null"} to int"),
     };