refactor: improving readability DecimalToAnyUsingStack

[alxkm]

• Extracted digit lookup table to a private static final field to avoid redundant creation.
• Added comprehensive JavaDoc for the class, method, and fields to improve readability and maintainability.
• Improved variable naming for clarity (tables → DIGITS, bits → digitStack).
• Minor code clean-up for better structure and readability.

• I have read CONTRIBUTING.md.
• This pull request is all my own work -- I have not plagiarized it.
• All filenames are in PascalCase.
• All functions and variable names follow Java naming conventions.
• All new algorithms have a URL in their comments that points to Wikipedia or other similar explanations.
• All new code is formatted with clang-format -i --style=file path/to/your/file.java

[codecov-commenter]

Codecov Report

Attention: Patch coverage is 86.66667% with 2 lines in your changes missing coverage. Please review.

Project coverage is 74.63%. Comparing base (ef93cc1) to head (d73e66f).

Files with missing lines	Patch %	Lines
...ms/dynamicprogramming/SubsetSumSpaceOptimized.java	75.00%	1 Missing and 1 partial ⚠️

Additional details and impacted files

@@             Coverage Diff              @@
##             master    #6377      +/-   ##
============================================
- Coverage     74.64%   74.63%   -0.01%     
- Complexity     5445     5446       +1     
============================================
  Files           682      682              
  Lines         19000    19001       +1     
  Branches       3668     3668              
============================================
- Hits          14182    14181       -1     
- Misses         4260     4261       +1     
- Partials        558      559       +1     

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