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

[tomas-zijdemans]

Adds serialization support to RollingCounter with toJSON(), RollingCounter.from(), and a RollingCounterSnapshot type. This enables JSON round-tripping and restoring persisted counter state

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 94.41%. Comparing base (1cd63ca) to head (6b4beb4).

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #7074   +/-   ##
=======================================
  Coverage   94.41%   94.41%           
=======================================
  Files         630      630           
  Lines       50490    50511   +21     
  Branches     8949     8955    +6     
=======================================
+ Hits        47669    47691   +22     
  Misses       2249     2249           
+ Partials      572      571    -1     

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