RFR: 8366224: Introduce DecimalDigits.appendPair for efficient two-digit formatting and refactor DateTimeHelper [v10]

[Shaojin Wen]

> This PR introduces a new efficient API for appending two-digit integers to StringBuilders and refactors DateTimeHelper to leverage this new functionality.
> 
> Changes include:
> 
> 1. 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
> 
> 2. 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.

Shaojin Wen has updated the pull request with a new target base due to a merge or a rebase. The pull request now contains 12 commits:

 - Merge remote-tracking branch 'upstream/master' into appendPair_202508
 - Merge remote-tracking branch 'upstream/master' into appendPair_202508
 - appendPair instead of appendLatin1(char, char)
 - Revert "appendPair instead of append(char, char)"
   
   This reverts commit d969a235e54da4ac96b7205982624f52305cae31.
 - appendPair instead of append(char, char)
 - Update test/jdk/jdk/internal/util/DecimalDigitsTest.java
   
   Co-authored-by: Chen Liang <liach at openjdk.org>
 - Merge remote-tracking branch 'upstream/master' into appendPair_202508
   
   # Conflicts:
   #	src/java.base/share/classes/jdk/internal/access/JavaLangAccess.java
 - Add DecimalDigitsTest to verify appendPair method with LATIN1 and UTF16 encoding
 - Use DecimalDigits.appendPair for formatting in time classes
   
   This change modifies the toString() methods in MonthDay, YearMonth, ZoneOffset, and ChronoLocalDateImpl to use DecimalDigits.appendPair for formatting two-digit numbers. This provides a more efficient and consistent way to format these values.
   
   Also added a comment in ChronoLocalDateImpl.toString() to explain why get() is used instead of getLong() for performance reasons, as the values are guaranteed to be within the int range for all chronologies.
   
   Co-authored-by: Qwen-Coder <qwen-coder at alibabacloud.com>
 - Optimize year formatting in DateTimeHelper by reducing modulo operation
   
   Co-authored-by: Qwen-Coder <qwen-coder at alibabacloud.com>
   
   Refactored the year formatting logic to use subtraction instead of modulo for calculating the lower two digits, which can be slightly more efficient. Added a comment to clarify the safety of the input range for DecimalDigits.appendPair.
 - ... and 2 more: https://git.openjdk.org/jdk/compare/02a72174...678e3066

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Changes: https://git.openjdk.org/jdk/pull/26911/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=26911&range=09
  Stats: 192 lines in 10 files changed: 161 ins; 4 del; 27 mod
  Patch: https://git.openjdk.org/jdk/pull/26911.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/26911/head:pull/26911

PR: https://git.openjdk.org/jdk/pull/26911