First of all:
This is a nice and simple solution and I really like it!

Issue:
Using this fine library I tried to draw a path that contained a point with the Y-coordinate 9.62E-4.
In the created SVG the Y-coordinate for said point was set to 9.62 instead, which lead to a unrecognizable Glyph in the result.

Observations:
Following this, I tried to find the cause of the issue and ended up playing arround with the class "RyuDouble" at the end.
As far as I understood it, Ryu is a method to create a String representing a floating point number (with increased speed and performance, when compared to other methods) and as far as I could see your intention was, to not only implement that method, but to also limit the maximum number of fractional digits to 4.

I tried to pinpoint my issue, by comparing the results of "RyuDouble.doubleToString(double value, int decimals)" to those of a simple DecimalFormat (which is simply dropping fractional digits beyond 4):

public static String doubleToString(double value, int decimals) {
   DecimalFormat decimalFormat = new DecimalFormat("#.#");
   decimalFormat.setMaximumFractionDigits(decimals);

   DecimalFormatSymbols decimalFormatSymbols = new DecimalFormatSymbols();
   decimalFormatSymbols.setDecimalSeparator('.');
   decimalFormat.setDecimalFormatSymbols(decimalFormatSymbols);

   return decimalFormat.format(value);
}

My results were as follows:

>>> LIMIT ALL VALUES TO 4 FRACTION DIGITS 
VALUE: '0'
   RYU_DOUBLE: '0.0'
   DECIMAL_FORMAT: '0'

VALUE: '0.000000000001'
   RYU_DOUBLE: '1.'
   DECIMAL_FORMAT: '0'

VALUE: '-1'
   RYU_DOUBLE: '-1.0'
   DECIMAL_FORMAT: '-1'

VALUE: '-0.0001'
   RYU_DOUBLE: '-1.0E-'
   DECIMAL_FORMAT: '-0.0001'

VALUE: '0.0001'
   RYU_DOUBLE: '1.0E-'
   DECIMAL_FORMAT: '0.0001'

VALUE: '0.01'
   RYU_DOUBLE: '0.01'
   DECIMAL_FORMAT: '0.01'

VALUE: '1000'
   RYU_DOUBLE: '1000.0'
   DECIMAL_FORMAT: '1000'

VALUE: '10000'
   RYU_DOUBLE: '10000.0'
   DECIMAL_FORMAT: '10000'

VALUE: '1000000000'
   RYU_DOUBLE: '1.0E9'
   DECIMAL_FORMAT: '1000000000'

Issues with "RyuDouble":

1. Some of the results yielded by "RyuDouble.doubleToString" are clearly incorrect or incomplete. (i.e. results for values '0.0001' and '0.000000000001')

2. I am not entirely sure, whether SVG does support the scientific notation at all!? Hence I did enforce a "raw number format" in my approach.

3. Even though I could find a recommendation to only use 2 fractional digits in SVG paths, I don't know whether I like the intended loss of precision (4 fractional digits). Is it really necessary to do this? I tried replacing "doubleToString" in "SVGGraphics2D" with my primitive approach - not limiting the maximum number of fractional digits - and the resulting SVG displayed just fine. (If it is necessary, I would rather round values?)

Assumption:
I did not read the paper concerning the Ryu method, therefore I can not claim this to be correct: But I would assume, that the combination of the Ryu method and the limitting of the fractional digits may cause the issue here.