Implement case-insensitive sorting for Acknowledgement

[Copilot]

The Acknowledgement type's Comparable implementation was case-sensitive, conflicting with the intended case-insensitive behavior and causing inconsistency with documentation examples.

Changes:

• Acknowledgement.Comparable: Use localizedStandardCompare(_:) for case-insensitive comparison (more performant than .lowercased())
• Acknowledgement.all(): Simplify to use .sorted() directly, delegating to Comparable
• Tests: Rename testSortingIsCaseSensitive → testSortingIsCaseInsensitive with updated expectations
• Documentation: Update improvement-plan.md example to reflect case-insensitive behavior

Before:

extension Acknowledgement: Comparable {
    public static func < (lhs: Acknowledgement, rhs: Acknowledgement) -> Bool {
        lhs.title < rhs.title  // Case-sensitive: "Zebra" < "apple"
    }
}

After:

extension Acknowledgement: Comparable {
    public static func < (lhs: Acknowledgement, rhs: Acknowledgement) -> Bool {
        lhs.title.localizedStandardCompare(rhs.title) == .orderedAscending  // "apple" < "Zebra"
    }
}

Sorting now correctly orders: ["apple", "Banana", "Zebra"] regardless of capitalization.

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[MartinP7r]

Closing in favor of #41 which applies the same fix rebased onto current main (the target branch feature/bug-fixes-and-tests was already merged via #31). All 16 tests pass.