Add custom slot resolvers via .custom(Resolver.self)

[kylebshr]

Summary

• Adds SlotResolver protocol allowing consumers to define custom slot options that map input types to views
• The @Slot(.custom(Resolver.self)) annotation generates constrained extensions using Resolver.Input, Resolver.Output, and Resolver.makeView()
• Supports .unlabeled modifier (.custom(R.self).unlabeled) and composition with built-in options (.text, .custom(R.self))
• Includes a Priority → PriorityBadge example in SlotExamples

Example usage

// Define a resolver
enum PriorityResolver: SlotResolver {
    typealias Input = Priority
    typealias Output = PriorityBadge
    static func makeView(_ input: Priority) -> PriorityBadge {
        PriorityBadge(priority: input)
    }
}

// Use it in a component
@Slots struct TaskRow<Title: View, Badge: View>: View {
    @Slot(.text) var title: Title
    @Slot(.custom(PriorityResolver.self)) var badge: Badge?
}

// Call sites
TaskRow(title: "Buy groceries", badge: .high)
TaskRow(title: "No priority")
TaskRow(title: "Custom badge") { Image(systemName: "star.fill") }

Test plan

• 4 new macro expansion tests (basic, optional, unlabeled, composition with built-in)
• Existing tests pass (including restored .text.unlabeled test)
• Example compiles and builds successfully

🤖 Generated with Claude Code