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.

tests/NATS.Server.Tests/NatsOptionsTests.cs

@@ -15,3 +15,24 @@ public class NatsOptionsTests
         opts.Tags.ShouldBeNull();
     }
 }
+
+public class LogOverrideTests
+{
+    [Fact]
+    public void LogOverrides_defaults_to_null()
+    {
+        var options = new NatsOptions();
+        options.LogOverrides.ShouldBeNull();
+    }
+
+    [Fact]
+    public void LogOverrides_can_be_set()
+    {
+        var options = new NatsOptions
+        {
+            LogOverrides = new() { ["NATS.Server.Protocol"] = "Trace" }
+        };
+        options.LogOverrides.ShouldNotBeNull();
+        options.LogOverrides["NATS.Server.Protocol"].ShouldBe("Trace");
+    }
+}