feat: add SlidingWindowAggregator, a ring buffer with windowed aggregation

[alxkm]

A fixed-capacity circular buffer that additionally keeps an aggregate of every element it currently holds, so a sliding window sum, minimum or maximum is available in constant time.

Rather than the usual "add the new value, subtract the evicted one" trick, which only works for invertible operators, it uses the two-stack sliding window aggregation algorithm: the window is split into a front section holding suffix aggregates and a back section summarised by a single running aggregate. Every element takes part in at most one rotation between the two, which makes insertion and removal O(1) amortised with no allocation after construction. Any associative operator works, commutative or not, and no identity element is needed.

Comes with factories for sum, minimum and maximum, indexed access, a fail-fast iterator, and tests that check the aggregate against a brute force reference under randomly interleaved insertions and removals.

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• 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.
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• All new algorithms include a corresponding test class that validates their functionality.
• All new code is formatted with clang-format -i --style=file path/to/your/file.java

[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 80.74%. Comparing base (0a6c756) to head (e3c7798).

Additional details and impacted files

@@             Coverage Diff              @@
##             master    #7591      +/-   ##
============================================
+ Coverage     80.68%   80.74%   +0.06%     
- Complexity     7529     7573      +44     
============================================
  Files           817      818       +1     
  Lines         24162    24257      +95     
  Branches       4759     4772      +13     
============================================
+ Hits          19494    19587      +93     
- Misses         3906     3907       +1     
- Partials        762      763       +1     

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[DenizAltunkapan]

@alxkm thanks!