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

❌ Patch coverage is 95.00000% with 1 line in your changes missing coverage. Please review.
✅ Project coverage is 94.57%. Comparing base (f0c9f14) to head (55d1f71).

Files with missing lines	Patch %	Lines
data_structures/unstable_rolling_counter.ts	95.00%	0 Missing and 1 partial ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #7161      +/-   ##
==========================================
- Coverage   94.57%   94.57%   -0.01%     
==========================================
  Files         636      636              
  Lines       52138    52152      +14     
  Branches     9399     9404       +5     
==========================================
+ Hits        49311    49324      +13     
  Misses       2249     2249              
- Partials      578      579       +1     

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