BUG: fmt.Errorf chain '%w: %v' loses original cause in embedded loader

### Describe the bug

In i18n/loader.go:15:
```go
return fmt.Errorf("%w: embedded load failed: %v", ErrLoadFailed, err)
```

Same pattern as the parser: the underlying load error is stringified and dropped from the chain.

### Expected behavior

Wrap both errors with `%w`.