Strengthen sortBy/maxBy/minBy tests with two distinct projections

[Copilot]

Tests for sortBy, maxBy, and minBy were using id as the projection, which can't detect bugs where the projection is silently ignored (e.g., sortBy behaving as sort).

Each function now has two property-based tests using opposite projections:

// identity projection — ascending order / max / min
NonEmptyList.sortBy id nel = List.sortBy id list
NonEmptyList.maxBy id nel = List.max list
NonEmptyList.minBy id nel = List.min list

// negation projection — descending order / min / max
NonEmptyList.sortBy (fun x -> -x) nel = List.sortBy (fun x -> -x) list
NonEmptyList.maxBy (fun x -> -x) nel = List.min list   // projection flips comparison
NonEmptyList.minBy (fun x -> -x) nel = List.max list

The negation projection verifies the projection is genuinely applied, since a correct maxBy (fun x -> -x) must return the minimum element.

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