feat(data-structures/unstable): add Readonly view and clone path to RollingCounter

[tomas-zijdemans]

Two additive changes, each adopting a pattern already used by sibling data structures.

• Add ReadonlyRollingCounter, a Pick<> view of the non-mutating members. RollingCounter now implements ReadonlyRollingCounter for a compile-time guarantee. Matches ReadonlyDeque and ReadonlyIndexedHeap.
• Widen RollingCounter.from() to accept a RollingCounter in addition to a RollingCounterSnapshot. The instance path produces an independent clone with a canonical layout (segments oldest-to-newest, cursor at the last position). Matches Deque.from(deque) and IndexedHeap.from(otherHeap).

No behavior changes for existing call sites.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 94.84%. Comparing base (0e5b1f2) to head (1f70590).

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #7161   +/-   ##
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  Coverage   94.83%   94.84%           
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  Files         617      618    +1     
  Lines       51674    51685   +11     
  Branches     9350     9352    +2     
=======================================
+ Hits        49007    49019   +12     
  Misses       2121     2121           
+ Partials      546      545    -1     

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[bartlomieju]

Clean, idiomatic, and well-tested — it mirrors the ReadonlyDeque / Deque.from(deque) precedent member-for-member. I verified the clone equals from(source.toJSON()), is fully independent (#segments reallocated, #total copied), the Pick includes exactly the non-mutating members, and the whole suite + doc examples pass on the branch. The error-message change ("snapshot" → "source") is safe since the throw tests assert error types, not text.

Approving. Two optional polish notes inline; neither is a blocker.