fix(config): stop mutating shared config structs during parsing

Prevent data loss and nil panics across 5 parse functions:
- chain: nil-guard hop entries to avoid deref on empty YAML lists
- hop: build merged metadata map instead of writing inherited values
  into the original NodeConfig; swap+restore around ParseNode call
- node: use local connCfg/dialCfg instead of overwriting cfg.Connector
  and cfg.Dialer fields; only fill in Type when empty
- parser: preserve existing config file fields when env vars override
  Level/Addr (was replacing the entire struct)
- router: only update ipNet from dst CIDR if parse succeeds, keeping
  the route.Net fallback instead of overwriting with nil

config/parsing/router/parse.go

@@ -51,7 +51,9 @@ func ParseRouter(cfg *config.RouterConfig) router.Router {
 		_, ipNet, _ := net.ParseCIDR(route.Net)
 		dst := route.Dst
 		if dst != "" {
-			_, ipNet, _ = net.ParseCIDR(dst)
+			if _, parsed, _ := net.ParseCIDR(dst); parsed != nil {
+				ipNet = parsed
+			}
 		} else {
 			if ipNet != nil {
 				dst = ipNet.String()