fix(router): close httpLoader on shutdown, log parse errors, default nil logger

- Add missing httpLoader.Close() in localRouter.Close() to prevent
  resource leak
- Log parse errors from file/redis/http loaders instead of silently
  discarding them
- Add nil reader guards before calling parseRoutes in load()
- Default nil logger to xlogger.Nop() to prevent nil panics
- Use context.Background() instead of context.TODO() for root context
- Use strings.ReplaceAll instead of deprecated strings.Replace with -1
- Add doc comments to all exported symbols
- Add 27 unit tests + 1 benchmark

router/router_other.go

@@ -6,6 +6,7 @@ import (
 	"github.com/go-gost/core/router"
 )
 
+// setSysRoutes is a no-op on non-Linux platforms.
 func (*localRouter) setSysRoutes(routes ...*router.Route) error {
 	return nil
 }