[Repo Assist] feat: add map, filter, iter, exists, forall, toArray to DList

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Summary

Adds six new module-level functions to DList, mirroring the functions added to Queue and Deque in #241, and further addressing #152 (aligning collection module functions).

Function	Description
DList.map	Transforms all elements using a mapping function
DList.filter	Retains only elements matching a predicate
DList.iter	Applies an action to each element in order
DList.exists	Returns true if any element satisfies the predicate (short-circuits)
DList.forall	Returns true if all elements satisfy the predicate (short-circuits)
DList.toArray	Returns elements as an array

Implementation Notes

• map and filter are implemented via foldBack, building a new DList directly. This preserves the length invariant (the int length field tracked in the DList type) without a separate counting pass.
• iter, exists, forall, and toArray delegate to the IEnumerable<'T> implementation. This gives correct short-circuit behaviour for exists/forall at no extra cost.

Tests

Adds 21 new tests:

• 14 unit tests covering normal cases, empty-list edge cases, and boundary conditions for all six functions.
• 7 property-based tests using testPropertyWithConfig generators (matching those used elsewhere in DListTest.fs) that verify results match the equivalent List/Array standard-library functions.

Test Status

Passed!  - Failed: 0, Passed: 727, Skipped: 6, Total: 733

All tests pass. Code formatted with Fantomas before commit.

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