feat: add per-subsystem log control via --log_level_override CLI flag

Adds LogOverrides property to NatsOptions and a --log_level_override=namespace=level CLI flag that wires Serilog MinimumLevel.Override entries so operators can tune verbosity per .NET namespace without changing the global log level.

src/NATS.Server/NatsOptions.cs

@@ -90,5 +90,8 @@ public sealed class NatsOptions
     public OcspConfig? OcspConfig { get; set; }
     public bool OcspPeerVerify { get; set; }
 
+    // Per-subsystem log level overrides (namespace -> level)
+    public Dictionary<string, string>? LogOverrides { get; set; }
+
     public bool HasTls => TlsCert != null && TlsKey != null;
 }