feat(data-structures/unstable): align IndexedHeap with heap APIs

[tomas-zijdemans]

• Rework unstable IndexedHeap around set() upsert semantics, replacing update() and pushOrUpdate().
• Add constructor loading and compare support, aligning IndexedHeap.from() behavior with other heap APIs.
• Generalize priorities beyond number, enabling max-heaps, bigint, and tuple priorities for stable tie-breaking.
• Expand docs and tests around the grouped unstable API changes.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 94.62%. Comparing base (a496da2) to head (d82df6e).

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #7124   +/-   ##
=======================================
  Coverage   94.61%   94.62%           
=======================================
  Files         634      634           
  Lines       51801    51824   +23     
  Branches     9329     9335    +6     
=======================================
+ Hits        49011    49036   +25     
  Misses       2216     2216           
+ Partials      574      572    -2     

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