[Repo Assist] feat: add fold, map, filter, iter, exists, forall, toList, toArray, ofList, ofArray, choose to Heap module

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Summary

Adds 11 missing collection-style functions to the Heap module, addressing the gap in #152 ("Align Collection Module functions with FSharp.Collections"):

Function	Description
Heap.toList	O(n) — returns elements as a sorted list
Heap.toArray	O(n) — returns elements as a sorted array
Heap.ofList	O(n log n) — builds a heap from a list
Heap.ofArray	O(n log n) — builds a heap from an array
Heap.fold	O(n) — folds over elements in sorted order
Heap.iter	O(n) — iterates over elements in sorted order
Heap.exists	O(n) — tests whether any element satisfies a predicate
Heap.forall	O(n) — tests whether all elements satisfy a predicate
Heap.map	O(n log n) — maps elements to a new heap, preserving sort direction
Heap.filter	O(n log n) — filters elements to a new heap
Heap.choose	O(n log n) — applies an option-returning function; keeps Some values

All functions are implemented as thin inline wrappers around the existing toSeq/ofSeq primitives. The sort direction (isDescending) is preserved automatically by map, filter, and choose.

Tests

18 new tests added to HeapTest.fs covering every new function including edge cases (empty heap, sort direction preservation, round-trips).

Test Status

✅ Build succeeded (0 errors, 25 warnings — all pre-existing)
✅ dotnet test — 757 passed, 0 failed, 6 skipped (pre-existing skips)
✅ Fantomas formatting check passed

Closes part of #152

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