This PR introduces a new efficient API for appending two-digit integers to StringBuilders and refactors DateTimeHelper to leverage this new functionality.

Changes include:

New appendPair method for efficient two-digit integer formatting (00-99):

Added AbstractStringBuilder.appendPair(int i) with core implementation
Added JavaLangAccess.appendPair(StringBuilder, int) for internal access
Added System.JavaLangAccessImpl.appendPair(StringBuilder, int) bridge
Added DecimalDigits.appendPair(StringBuilder, int) public static utility method
Enhanced Javadoc documentation for all new methods
Refactored DateTimeHelper to use the new DecimalDigits.appendPair:

Updated DateTimeHelper.formatTo methods for LocalDate and LocalTime
Replaced manual formatting logic with the new efficient two-digit appending
Improved code clarity and consistency in date/time formatting
These changes improve code clarity and performance when formatting two-digit numbers, particularly in date/time formatting scenarios.