feat: AppKit renderer (A2UIAppKit)

[BBC6BAE9]

Background

A2UISwiftCore is completely UI-framework-agnostic. It has zero dependency on SwiftUI, UIKit, or AppKit — making it suitable for integration with any Apple UI framework, including SwiftUI, UIKit, AppKit, and server-side Swift.

Current State

The existing A2UISwiftUI target demonstrates a full renderer built on top of A2UISwiftCore. The data layer is already ready for AppKit:

• DataContext.subscribeDynamicValue() provides reactive data subscriptions without any SwiftUI dependency
• MessageProcessor and SurfaceViewModel handle all protocol-level message processing
• ComponentNode tree is framework-agnostic and can be walked by any renderer

What Needs to Be Done

• Add a new A2UIAppKit SPM target in Package.swift with A2UISwiftCore as its only dependency
• Implement a component set parallel to A2UISwiftUI, using native AppKit controls (NSTextField, NSButton, NSStackView, etc.)
• Use DataContext.subscribeDynamicValue() to drive UI updates reactively
• Handle component lifecycle (subscribe when view appears, unsubscribe when view deallocates)

Where to Start

1. Read through Sources/A2UISwiftUI/ — the data layer calls are identical, only the rendering changes
2. If feat: UIKit renderer (A2UIUIKit) #24 (UIKit renderer) is already in progress, consider referencing its architecture — UIKit and AppKit renderers share the same data-layer patterns
3. Start with the four core layout components: Text, Button, Column, Row
4. Validate the architecture with those four before expanding to the full component set