length: count runes, not bytes

MinLength/MaxLength measured len(value), so a string of 8 accented
characters (9+ bytes) failed MaxLength(8) despite the error message
promising "characters". Count with utf8.RuneCountInString so the check
matches the wording and the common-sense intent.

Still not grapheme-cluster aware (combining marks, emoji ZWJ sequences
count as several runes), which is out of scope for a stdlib-only helper.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

length.go

@@ -1,15 +1,17 @@
 package validate
 
+import "unicode/utf8"
+
 var (
 	ErrMustBeLonger  = NewError("must contain at least %d characters")
 	ErrMustBeShorter = NewError("must contain no more than %d characters")
 )
 
 // MaxLength validates that a string is no longer than a given maximum.
-// Length is measured in bytes (len), not runes.
+// Length is counted in runes, so multi-byte characters count as one.
 func MaxLength(l int) func(string) error {
 	return func(value string) error {
-		if len(value) > l {
+		if utf8.RuneCountInString(value) > l {
 			return ErrMustBeShorter.With(l)
 		}
 		return nil
@@ -17,10 +19,10 @@ func MaxLength(l int) func(string) error {
 }
 
 // MinLength validates that a string is at least a given minimum length.
-// Length is measured in bytes (len), not runes.
+// Length is counted in runes, so multi-byte characters count as one.
 func MinLength(l int) func(string) error {
 	return func(value string) error {
-		if len(value) < l {
+		if utf8.RuneCountInString(value) < l {
 			return ErrMustBeLonger.With(l)
 		}
 		return nil