Add JSONValue and UncheckedSendable

[stevenzeck]

In order to make the Swift 6 migration a little less painful, I wanted to extract 3 file changes separately. These are core to replacing the use of Any in json with type safe.

This doesn't break any functionality as they are simple additions.

[Copilot]

Pull request overview

This PR introduces new Swift 6-migration helpers to replace untyped JSON (Any) with a type-safe JSONValue, plus an UncheckedSendable wrapper to ease Sendable adoption.

Changes:

• Add JSONValue (Sendable/Hashable, literals, Codable) and ReadResult JSON decoding helpers returning JSONValue.
• Add UncheckedSendable<T> wrapper for explicitly opting into @unchecked Sendable.
• Add unit tests for JSONValue and update Carthage/Xcode project generation files to include the new sources.

Reviewed changes

Copilot reviewed 6 out of 6 changed files in this pull request and generated 5 comments.

Show a summary per file

File	Description
Tests/SharedTests/Toolkit/JSONValueTests.swift	Adds test coverage for JSONValue initialization, literals, Codable, and decoding helpers.
Sources/Shared/Toolkit/JSONValue.swift	Introduces the type-safe JSON representation and conversions from Any, plus literal/Codable conformances.
Sources/Shared/Toolkit/UncheckedSendable.swift	Adds a wrapper type to force Sendable conformance when the caller guarantees thread-safety.
Sources/Shared/Toolkit/JSON.swift	Adds ReadResult-style helpers to decode Data/Data? into JSONValue / object dictionaries.
Support/Carthage/Readium.xcodeproj/project.pbxproj	Includes the new Swift source files in the Carthage Xcode project build.
Support/Carthage/.xcodegen	Updates the XcodeGen input list (currently includes unwanted .DS_Store paths).

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