feat: integrate config file loading and SIGHUP hot reload

Wire up the config parsing infrastructure into the server:
- NatsServer: add ReloadConfig() with digest-based change detection,
  diff/validate, CLI override preservation, and side-effect triggers
- Program.cs: two-pass CLI parsing — load config file first, then
  apply CLI args on top with InCmdLine tracking for reload precedence
- SIGHUP handler upgraded from stub warning to actual reload
- Remove config file "not yet supported" warning from StartAsync
- Add integration tests for config loading, CLI overrides, and
  reload validation

src/NATS.Server.Host/Program.cs

@@ -1,85 +1,124 @@
 using NATS.Server;
+using NATS.Server.Configuration;
 using Serilog;
 using Serilog.Sinks.SystemConsole.Themes;
 
-var options = new NatsOptions();
+// First pass: scan args for -c flag to get config file path
+string? configFile = null;
+for (int i = 0; i < args.Length; i++)
+{
+    if (args[i] == "-c" && i + 1 < args.Length)
+    {
+        configFile = args[++i];
+        break;
+    }
+}
 
-// Parse ALL CLI flags into NatsOptions first
+// If config file specified, load it as the base options
+var options = configFile != null
+    ? ConfigProcessor.ProcessConfigFile(configFile)
+    : new NatsOptions();
+
+// Second pass: apply CLI args on top of config-loaded options, tracking InCmdLine
 for (int i = 0; i < args.Length; i++)
 {
     switch (args[i])
     {
         case "-p" or "--port" when i + 1 < args.Length:
             options.Port = int.Parse(args[++i]);
+            options.InCmdLine.Add("Port");
             break;
         case "-a" or "--addr" when i + 1 < args.Length:
             options.Host = args[++i];
+            options.InCmdLine.Add("Host");
             break;
         case "-n" or "--name" when i + 1 < args.Length:
             options.ServerName = args[++i];
+            options.InCmdLine.Add("ServerName");
             break;
         case "-m" or "--http_port" when i + 1 < args.Length:
             options.MonitorPort = int.Parse(args[++i]);
+            options.InCmdLine.Add("MonitorPort");
             break;
         case "--http_base_path" when i + 1 < args.Length:
             options.MonitorBasePath = args[++i];
+            options.InCmdLine.Add("MonitorBasePath");
             break;
         case "--https_port" when i + 1 < args.Length:
             options.MonitorHttpsPort = int.Parse(args[++i]);
+            options.InCmdLine.Add("MonitorHttpsPort");
             break;
         case "-c" when i + 1 < args.Length:
-            options.ConfigFile = args[++i];
+            // Already handled in first pass; skip the value
+            i++;
             break;
         case "--pid" when i + 1 < args.Length:
             options.PidFile = args[++i];
+            options.InCmdLine.Add("PidFile");
             break;
         case "--ports_file_dir" when i + 1 < args.Length:
             options.PortsFileDir = args[++i];
+            options.InCmdLine.Add("PortsFileDir");
             break;
         case "--tls":
             break;
         case "--tlscert" when i + 1 < args.Length:
             options.TlsCert = args[++i];
+            options.InCmdLine.Add("TlsCert");
             break;
         case "--tlskey" when i + 1 < args.Length:
             options.TlsKey = args[++i];
+            options.InCmdLine.Add("TlsKey");
             break;
         case "--tlscacert" when i + 1 < args.Length:
             options.TlsCaCert = args[++i];
+            options.InCmdLine.Add("TlsCaCert");
             break;
         case "--tlsverify":
             options.TlsVerify = true;
+            options.InCmdLine.Add("TlsVerify");
             break;
         case "-D" or "--debug":
             options.Debug = true;
+            options.InCmdLine.Add("Debug");
             break;
         case "-V" or "-T" or "--trace":
             options.Trace = true;
+            options.InCmdLine.Add("Trace");
             break;
         case "-DV":
             options.Debug = true;
             options.Trace = true;
+            options.InCmdLine.Add("Debug");
+            options.InCmdLine.Add("Trace");
             break;
         case "-l" or "--log" or "--log_file" when i + 1 < args.Length:
             options.LogFile = args[++i];
+            options.InCmdLine.Add("LogFile");
             break;
         case "--log_size_limit" when i + 1 < args.Length:
             options.LogSizeLimit = long.Parse(args[++i]);
+            options.InCmdLine.Add("LogSizeLimit");
             break;
         case "--log_max_files" when i + 1 < args.Length:
             options.LogMaxFiles = int.Parse(args[++i]);
+            options.InCmdLine.Add("LogMaxFiles");
             break;
         case "--logtime" when i + 1 < args.Length:
             options.Logtime = bool.Parse(args[++i]);
+            options.InCmdLine.Add("Logtime");
             break;
         case "--logtime_utc":
             options.LogtimeUTC = true;
+            options.InCmdLine.Add("LogtimeUTC");
             break;
         case "--syslog":
             options.Syslog = true;
+            options.InCmdLine.Add("Syslog");
             break;
         case "--remote_syslog" when i + 1 < args.Length:
             options.RemoteSyslog = args[++i];
+            options.InCmdLine.Add("RemoteSyslog");
             break;
     }
 }
@@ -136,6 +175,14 @@ Log.Logger = logConfig.CreateLogger();
 using var loggerFactory = new Serilog.Extensions.Logging.SerilogLoggerFactory(Log.Logger);
 using var server = new NatsServer(options, loggerFactory);
 
+// Store CLI snapshot for reload precedence (CLI flags always win over config file)
+if (configFile != null && options.InCmdLine.Count > 0)
+{
+    var cliSnapshot = new NatsOptions();
+    ConfigReloader.MergeCliOverrides(cliSnapshot, options, options.InCmdLine);
+    server.SetCliSnapshot(cliSnapshot, options.InCmdLine);
+}
+
 // Register signal handlers
 server.HandleSignals();