feat(data-structures/unstable): add RollingCounter.at() and toArray()

[tomas-zijdemans]

This PR is mostly about aligning RollingCounter with Deque, IndexedHeap, MultiMap and other data-structures.

• RollingCounter now implements Iterable.
• [Symbol.toStringTag] is a readonly "RollingCounter" as const property (was a getter).
• @experimental tag added to every public-member JSDoc.
• Class JSDoc gained a complexity table.
• New at(index) with Array.prototype.at semantics (oldest→newest, negatives from newest end, out-of-range → undefined).
• New toArray(); toJSON() delegates to it.
• RollingCounterSnapshot.segments is readonly number[]; field is readonly.
• from() accepts RollingCounterSnapshot, guards null/undefined/non-object, throws TypeError for shape errors, keeps RangeError for per-element numeric errors.
• Tests: two RangeError cases flipped to TypeError; added null/undefined snapshot, toArray, and at tests.

[codecov]

Codecov Report

❌ Patch coverage is 97.05882% with 1 line in your changes missing coverage. Please review.
✅ Project coverage is 94.59%. Comparing base (f318978) to head (4b6e0b7).

Files with missing lines	Patch %	Lines
data_structures/unstable_rolling_counter.ts	97.05%	0 Missing and 1 partial ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #7102      +/-   ##
==========================================
- Coverage   94.59%   94.59%   -0.01%     
==========================================
  Files         632      632              
  Lines       51780    51797      +17     
  Branches     9324     9331       +7     
==========================================
+ Hits        48981    48997      +16     
  Misses       2225     2225              
- Partials      574      575       +1     

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