Drop Carthage support and adopt Swift Package Manager layout

[Econa77]

Summary

This ends the legacy Carthage-oriented repository layout and makes Swift Package Manager the primary project structure.

• Move the library sources from Lib/KeyHolder to Sources/KeyHolder and tests from Lib/KeyHolderTests to Tests/KeyHolderTests.
• Remove the old library Xcode project, workspace, Carthage files, and Magnet/Sauce submodules that were only needed for the previous framework-based setup.
• Simplify Package.swift to use the standard SwiftPM source and test target paths, and declare the direct Sauce dependency used by RecordView.
• Update the Example app to link the local KeyHolder Swift package product directly and resolve Magnet/Sauce through SwiftPM.
• Replace the README CI badge with the existing SwiftPM build workflow and drop the unused CocoaPods gem dependency.

This keeps the package focused on SwiftPM as the source of truth while preserving the Example app as a consumer of the local package.

Checked locally with swift test and xcodebuild -project Example/Example.xcodeproj -scheme Example -configuration Debug -derivedDataPath /private/tmp/KeyHolderDerivedDataReview CODE_SIGNING_ALLOWED=NO build.