// Package xerrors provides structured application errors with stable typed codes,
// cause chaining, and key-value context fields.
//
// Every error carries a machine-readable Code (gRPC-aligned wire value), a
// human-readable message, an optional cause, and optional structured fields.
// *Err implements errs.CodedError and errs.ContextualError from contracts,
// enabling logz to enrich log records automatically without importing this package.
//
// Usage:
//
//	// Named constructors for common codes
//	err := xerrors.NotFound("user %s not found", userID)
//	err := xerrors.InvalidInput("email is required")
//
//	// Builder pattern for structured context
//	err := xerrors.New(xerrors.ErrInvalidInput, "validation failed").
//	    WithContext("field", "email").
//	    WithContext("rule", "required").
//	    WithError(cause)
//
//	// Inspecting errors
//	var e *xerrors.Err
//	if errors.As(err, &e) {
//	    switch e.Code() {
//	    case xerrors.ErrNotFound:
//	        // handle 404
//	    }
//	}
package xerrors