feat: Add AnchoredPopup support for absolute positioning

[videni]

Summary

Add AnchoredPopup protocol and configuration for absolute positioning of popups, allowing popups to be anchored to specific screen coordinates.

Closes #197

Changes

• Add PopupAlignment.anchored case
• Create AnchoredPopup protocol and AnchoredPopupConfig
• Implement present(anchoredTo:) API for specifying anchor frame
• Create PopupAnchoredStackView for rendering anchored popups
• Support originAnchor/popupAnchor configuration for flexible positioning
• Add PopupAnchorPoint enum with 9 anchor positions

Usage

struct MyAnchoredPopup: AnchoredPopup {
    var body: some View {
        Text("Anchored content")
    }
    
    func configurePopup(config: AnchoredPopupConfig) -> AnchoredPopupConfig {
        config
            .originAnchor(.bottom)
            .popupAnchor(.top)
    }
}

// Present anchored to a specific frame
await MyAnchoredPopup().present(anchoredTo: buttonFrame)

AnchoredPopup Compatibility Limitations

AnchoredPopup is iOS/iPadOS only. Not available on macOS, tvOS, or watchOS.

Why This Limitation Exists

Core Reason: Touch Pass-Through Requires UIKit

AnchoredPopup's key feature is allowing touches outside the popup to pass through to underlying views. This requires:

Feature	UIKit Implementation	SwiftUI Alternative
Intercept touch events	point(inside:with:)	❌ None
Forward touches to layers below	hitTest(_:with:)	❌ None
Independent window layer	UIWindow subclass	❌ No precise control
Access window reference	UIViewRepresentable	NSViewRepresentable (macOS)

Why Pure SwiftUI Won't Work

Popup Layer (Popup Window)
↓ tap outside
Main View Layer (App Window) ← needs to receive event

SwiftUI's overlay / ZStack cannot:

• Intercept events in one area (inside popup)
• Pass through events in another area (outside popup)

UIKit's hitTest override enables precise per-point event routing.

Design Trade-offs

UIKit implementation was chosen for:

1. Feature completeness - Touch pass-through is essential for anchored popups in my case
2. Primary platform - iOS/iPadOS is the main target
3. UX requirements - Users need to interact with content beneath the popup without dismissing it

Future Options

Approach	Effort	Result
Conditional compilation (disable)	Low	No macOS support
Pure SwiftUI (simplified)	Medium	No touch pass-through
AppKit native implementation	High	Full functionality