length: add MinLengthBytes / MaxLengthBytes

MinLength/MaxLength now count runes, leaving no way to bound a string by
its byte size -- still wanted for fixed-width columns and wire-format
fields. Add the byte-counting pair alongside them, with their own "%d
bytes" sentinels (ErrMustHaveMoreBytes / ErrMustHaveFewerBytes).

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

length.go

@@ -3,8 +3,10 @@ package validate
 import "unicode/utf8"
 
 var (
-	ErrMustBeLonger  = NewError("must contain at least %d characters")
-	ErrMustBeShorter = NewError("must contain no more than %d characters")
+	ErrMustBeLonger       = NewError("must contain at least %d characters")
+	ErrMustBeShorter      = NewError("must contain no more than %d characters")
+	ErrMustHaveMoreBytes  = NewError("must have at least %d bytes")
+	ErrMustHaveFewerBytes = NewError("must have no more than %d bytes")
 )
 
 // MaxLength validates that a string is no longer than a given maximum.
@@ -28,3 +30,28 @@ func MinLength(l int) func(string) error {
 		return nil
 	}
 }
+
+// MaxLengthBytes validates that a string is no longer than a given maximum
+// number of bytes (len). Prefer [MaxLength] for a limit on visible
+// characters; use this when the budget is genuinely a byte count, such as
+// a fixed-width column or a wire-format field.
+func MaxLengthBytes(l int) func(string) error {
+	return func(value string) error {
+		if len(value) > l {
+			return ErrMustHaveFewerBytes.With(l)
+		}
+		return nil
+	}
+}
+
+// MinLengthBytes validates that a string is at least a given minimum
+// number of bytes (len). See [MaxLengthBytes] on when to prefer this over
+// [MinLength].
+func MinLengthBytes(l int) func(string) error {
+	return func(value string) error {
+		if len(value) < l {
+			return ErrMustHaveMoreBytes.With(l)
+		}
+		return nil
+	}
+}