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_test.go

@@ -73,3 +73,69 @@ func TestMinLength(t *testing.T) {
 		}
 	}
 }
+
+func ExampleMaxLengthBytes() {
+	testMaxLengthBytes := MaxLengthBytes(8)
+	fmt.Println(testMaxLengthBytes("cafés round the world"))
+	// Output: must have no more than 8 bytes
+}
+
+func ExampleMinLengthBytes() {
+	testMinLengthBytes := MinLengthBytes(8)
+	fmt.Println(testMinLengthBytes("2short"))
+	// Output: must have at least 8 bytes
+}
+
+func TestMaxLengthBytes(t *testing.T) {
+	testCases := map[int]map[string]error{
+		8: {
+			"abcd":      nil,
+			"abcdefgh":  nil,
+			"abcdéfg":   nil,                           // 7 runes, 8 bytes
+			"abcdéfgh":  ErrMustHaveFewerBytes.With(8), // 8 runes, 9 bytes
+			"abcdefghi": ErrMustHaveFewerBytes.With(8),
+		},
+	}
+
+	for setup, values := range testCases {
+		testMaxLengthBytes := MaxLengthBytes(setup)
+
+		for input, want := range values {
+			t.Run(fmt.Sprintf("%d/%s", setup, input), func(t *testing.T) {
+				got := testMaxLengthBytes(input)
+
+				if !errors.Is(got, want) {
+					t.Error("got", got)
+					t.Error("want", want)
+				}
+			})
+		}
+	}
+}
+
+func TestMinLengthBytes(t *testing.T) {
+	testCases := map[int]map[string]error{
+		8: {
+			"abcd":     ErrMustHaveMoreBytes.With(8),
+			"abcdefg":  ErrMustHaveMoreBytes.With(8),
+			"abcdéf":   ErrMustHaveMoreBytes.With(8), // 6 runes, 7 bytes
+			"abcdéfg":  nil,                          // 7 runes, 8 bytes
+			"abcdefgh": nil,
+		},
+	}
+
+	for setup, values := range testCases {
+		testMinLengthBytes := MinLengthBytes(setup)
+
+		for input, want := range values {
+			t.Run(fmt.Sprintf("%d/%s", setup, input), func(t *testing.T) {
+				got := testMinLengthBytes(input)
+
+				if !errors.Is(got, want) {
+					t.Error("got", got)
+					t.Error("want", want)
+				}
+			})
+		}
+	}
+}