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/

doc.go

@@ -0,0 +1,31 @@
+// Package valid provides struct validation backed by [github.com/go-playground/validator/v10].
+//
+// Create a validator with [New]:
+//
+//	v := valid.New()
+//
+// Validate a struct:
+//
+//	type CreateUserRequest struct {
+//	    Name  string `validate:"required"`
+//	    Email string `validate:"required,email"`
+//	    Age   int    `validate:"min=18,max=120"`
+//	}
+//
+//	err := v.Struct(req)
+//	if err != nil {
+//	    // err is a *xerrors.Err with code ErrInvalidInput.
+//	    // Use errors.As to inspect it.
+//	}
+//
+// Use Spanish messages:
+//
+//	v := valid.New(valid.WithMessageProvider(valid.SpanishMessages))
+//
+// Use a custom message provider:
+//
+//	type myMessages struct{}
+//	func (m myMessages) Message(field, tag, param string) string { ... }
+//
+//	v := valid.New(valid.WithMessageProvider(myMessages{}))
+package valid