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   +/-   ##
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  Coverage   94.61%   94.62%           
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  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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[fibibot]

The update/pushOrUpdate collapse into a single set() upsert matches Map's API and is the natural shape for Dijkstra-style relaxation. Generic priority P = number with the compare option keeps the default ergonomic for the common case and unlocks tuple/bigint/descend variants without internal forks.

from(otherHeap) correctly inherits #compare and reuses the source's heap-array order (still valid under the same comparator); the override branch re-heapifies via Floyd's siftDown in O(n). checkPriority narrowing to typeof === "number" && isNaN is right — NaN only exists in number.

Tests cover the new surface well: tuple stable tie-breaking, bigint past MAX_SAFE_INTEGER, set no-op on identical priority, NaN rejection on both absent and present keys, ArrayLike redirected to from().

• nit: from(otherHeap, { compare: collection.#compare }) re-heapifies even when the comparator is identical to the source's. Cheap edge case to skip, but function identity is fragile so probably not worth optimising.

[fibibot]

@bartlomieju this is ready to merge