fix(convert): zero-pad each channel in RgbToHex

RgbToHex emitted a single hex digit for channel values below 0x10 because
strconv.FormatInt produces the minimal representation. This broke the
fixed-width hex format, the round-trip with HexToRgb, and the hex2basicMap
fast path in Rgb2basic. Format each channel with %02x, matching RgbTo256.

convert.go

@@ -483,7 +483,10 @@ func RgbToHex(rgb []int) string {
 	hexNodes := make([]string, 0, len(rgb))
 
 	for _, v := range rgb {
-		hexNodes = append(hexNodes, strconv.FormatInt(int64(v), 16))
+		// each channel must be two hex digits, otherwise channels below 0x10
+		// are emitted as a single digit and break both the fixed-width hex
+		// format and the round-trip with HexToRgb.
+		hexNodes = append(hexNodes, fmt.Sprintf("%02x", v))
 	}
 	return strings.Join(hexNodes, "")
 }