Add View.tag API support

[Kyle-Ye]

Summary

Implements the View.tag(_:includeOptional:) API to enable tagging views for selection in containers like Picker and TabView.

Closes #583

Changes

Adds Sources/OpenSwiftUICore/View/Tag.swift with complete implementation:

Public API

• View.tag(_:includeOptional:): Sets unique tag values on views for identification in selectable containers
• View._untagged(): Marks views as auxiliary content that should not be wrapped by containers

Internal Components

• TagValueTraitKey<V>: Trait key for storing tag values with support for both tagged and untagged states
• IsAuxiliaryContentTraitKey: Trait key for marking auxiliary/untagged content
• ViewTraitCollection extensions: Methods for reading and writing tag values
  • tagValue(for:) - Get tag value regardless of auxiliary status
  • tag(for:) - Get tag value (returns nil for auxiliary content)
  • setTagIfUnset(for:value:) - Set tag only if not already set
  • setTag(for:value:) - Unconditionally set tag value
• Binding extensions: Tag-based binding projections
  • selecting(_:) - Create boolean binding for tag selection
  • projectingTagIndex(viewList:) - Project binding to tag index in view list
• TagIndexProjection: Internal projection class for efficient tag-to-index mapping

API Overview

extension View {
    func tag<V>(_ tag: V, includeOptional: Bool = true) -> some View where V: Hashable
    func _untagged() -> some View
}

Example Usage

struct FlavorPicker: View {
    enum Flavor: String, CaseIterable, Identifiable {
        case chocolate, vanilla, strawberry
        var id: Self { self }
    }
    
    @State private var selectedFlavor: Flavor = .chocolate
    
    var body: some View {
        Picker("Flavor", selection: $selectedFlavor) {
            ForEach(Flavor.allCases) { flavor in
                Text(flavor.rawValue)
                    .tag(flavor)
            }
        }
    }
}

Implementation Details

• Uses trait-based storage for tag values
• Supports both optional and non-optional tag types via includeOptional parameter
• Efficient tag-to-index mapping for large view lists

[codecov]

Codecov Report

❌ Patch coverage is 0% with 90 lines in your changes missing coverage. Please review.
✅ Project coverage is 32.02%. Comparing base (b5860b3) to head (3448b25).
⚠️ Report is 1 commits behind head on main.

Files with missing lines	Patch %	Lines
Sources/OpenSwiftUICore/View/Tag.swift	0.00%	90 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #584      +/-   ##
==========================================
- Coverage   32.11%   32.02%   -0.10%     
==========================================
  Files         526      527       +1     
  Lines       30650    30740      +90     
==========================================
  Hits         9844     9844              
- Misses      20806    20896      +90     

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