feat(valid): initial stable release v0.9.0

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/

messages.go

@@ -0,0 +1,61 @@
+package valid
+
+import "fmt"
+
+// MessageProvider maps a validation failure to a human-readable message.
+//
+//   - field: the struct field name (e.g. "Email")
+//   - tag:   the failing validation rule (e.g. "required", "email", "min")
+//   - param: the rule parameter if any (e.g. "18" for min=18), or "" if none
+type MessageProvider interface {
+	Message(field, tag, param string) string
+}
+
+// DefaultMessages is the built-in English message provider.
+// Used automatically when no WithMessageProvider option is given.
+var DefaultMessages MessageProvider = defaultMessages{}
+
+// SpanishMessages is an opt-in Spanish message provider.
+var SpanishMessages MessageProvider = spanishMessages{}
+
+// ---------------------------------------------------------------
+// English (default)
+// ---------------------------------------------------------------
+
+type defaultMessages struct{}
+
+func (defaultMessages) Message(field, tag, param string) string {
+	switch tag {
+	case "required":
+		return fmt.Sprintf("field '%s' is required", field)
+	case "email":
+		return fmt.Sprintf("field '%s' must be a valid email address", field)
+	case "min":
+		return fmt.Sprintf("field '%s' is too short (minimum %s)", field, param)
+	case "max":
+		return fmt.Sprintf("field '%s' is too long (maximum %s)", field, param)
+	default:
+		return fmt.Sprintf("field '%s' failed validation rule '%s'", field, tag)
+	}
+}
+
+// ---------------------------------------------------------------
+// Spanish (opt-in)
+// ---------------------------------------------------------------
+
+type spanishMessages struct{}
+
+func (spanishMessages) Message(field, tag, param string) string {
+	switch tag {
+	case "required":
+		return fmt.Sprintf("El campo '%s' es obligatorio", field)
+	case "email":
+		return fmt.Sprintf("El campo '%s' debe ser un correo electrónico válido", field)
+	case "min":
+		return fmt.Sprintf("El campo '%s' es demasiado corto (mínimo %s)", field, param)
+	case "max":
+		return fmt.Sprintf("El campo '%s' es demasiado largo (máximo %s)", field, param)
+	default:
+		return fmt.Sprintf("Error en el campo '%s': regla '%s' no cumplida", field, tag)
+	}
+}