feat(data-structures/unstable): add RollingCounter serialization

[tomas-zijdemans]

• Add toJSON() / RollingCounter.from() for JSON-compatible serialization and restoration
• Add current getter for reading the newest segment without iteration
• Add Symbol.toStringTag to match module precedent.
• Optimize rotate() (up to 5.5x faster for bulk rotations) by replacing per-element modulo with contiguous-range processing

[codecov]

Codecov Report

❌ Patch coverage is 98.30508% with 1 line in your changes missing coverage. Please review.
✅ Project coverage is 94.41%. Comparing base (1cd63ca) to head (9cded39).

Files with missing lines	Patch %	Lines
data_structures/unstable_rolling_counter.ts	98.30%	0 Missing and 1 partial ⚠️

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #7074   +/-   ##
=======================================
  Coverage   94.41%   94.41%           
=======================================
  Files         630      630           
  Lines       50490    50543   +53     
  Branches     8949     8963   +14     
=======================================
+ Hits        47669    47721   +52     
  Misses       2249     2249           
- Partials      572      573    +1     

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