feat(data-structures/unstable): add includes() and removeAt() to Deque

[tomas-zijdemans]

• Add includes() method (matching Array.prototype.includes), and removeAt(index) for removing elements by index with negative-index support.
• Add a time-complexity table to the Deque class JSDoc to match BinaryHeap docs.
• Optimize hot paths: replace generator-based [Symbol.iterator]() and reversed() with manual iterator objects, use slice/concat in toArray() and #copyBuffer instead of per-element loops, and add a fast path in Deque.from() for plain arrays without a map function
• Extract #maybeGrow() and #removeAtUnchecked() private helpers to reduce duplication

[codecov]

Codecov Report

❌ Patch coverage is 95.83333% with 4 lines in your changes missing coverage. Please review.
✅ Project coverage is 94.41%. Comparing base (1cd63ca) to head (bac0575).

Files with missing lines	Patch %	Lines
data_structures/unstable_deque.ts	95.83%	3 Missing and 1 partial ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #7083      +/-   ##
==========================================
- Coverage   94.41%   94.41%   -0.01%     
==========================================
  Files         630      630              
  Lines       50490    50551      +61     
  Branches     8949     8959      +10     
==========================================
+ Hits        47669    47726      +57     
- Misses       2249     2252       +3     
- Partials      572      573       +1     

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