Struct validation backed by go-playground/validator/v10 with xerrors integration and pluggable i18n message providers.
What's included:
- Validator interface with Struct(v any) error method
- New(...Option) constructor with WithMessageProvider functional option
- MessageProvider interface for i18n; DefaultMessages (EN) and SpanishMessages (ES) built in
- ValidationErrors mapped to xerrors.ErrInvalidInput with field and tag context keys
- InvalidValidationError (non-struct input) mapped to xerrors.ErrInternal
- Full playground.ValidationErrors attached via WithError for callers needing all failures
Tested-via: todo-api POC integration
Reviewed-against: docs/adr/
