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
